Urinary System of the Animals

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All vertebrates dispose of excess water and other wastes by means of kidneys. The kidneys of fish and amphibians are comparatively simple, while those of mammals are the most complex. Fish and amphibians absorb a great deal of water and, as a result, must excrete large quantities of urine. In contrast, the urinary systems of birds and reptiles are designed to conserve water; these animals produce urine that is solid or semisolid.

Reproductive System of Animals

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In many invertebrate species individual animals bear both testes and ovaries (see Hermaphroditism). In some invertebrates, and in most vertebrates, individuals bear either testes or ovaries, but not both sets of organs. In invertebrates, a single animal may have as many as 26 pairs of gonads; in vertebrates, the usual number is 2. Cyclostomes and most birds are unusual among vertebrates in possessing only a single gonad; owls, pigeons, hawks, and parrots are unusual among birds in having two gonads. The size of gonads increases at sexual maturity because of the great number of germ cells produced at that time; many germ cells are also produced during breeding seasons so that many animals have a seasonal increase in size of the gonads. During the breeding season of fish, the ovaries increase in size until they constitute about one-quarter to one-third of the total body weight.

The testes and ovaries of mature animals differ greatly in structure. The testes are composed of delicate convoluted tubules, known as seminiferous tubules, in which the primitive germ cells mature into spermatozoa. The testes of mammals are generally oval bodies, enclosed by a capsule of tough connective tissue. Projections from this tough capsule into the testis divide the testis into several compartments, each of which is filled with hundreds of seminiferous tubules. The mature spermatozoa are discharged through a number of ducts, called the efferent ducts, which communicate with the epididymis, a thick-walled, coiled duct in which the sperm are stored.

In all vertebrates below marsupials on the zoological scale, and in elephants, sea cows, and whales, the testis remains within the body cavity during the lifetime of the animals. In many mammals, such as rodents, bats, and members of the camel family, the testis remains within the body cavity during periods of quiescence, but moves into an external pocket of skin and muscle, known as the scrotum, during the breeding season. In marsupials, and in most higher mammals, including the human male, the testes are always enclosed in an external scrotum. During fetal life, the testes move through the muscles composing the posterior, ventral portion of the trunk and carry with them the portion of the peritoneum and skin surrounding these muscles. The channel in the muscles through which the testis moves is known as the inguinal canal; it usually closes after birth, but sometimes remains open and is then often the site of herniation (see Hernia). The portion of the peritoneum that the testis carries with it forms a double wall of membrane between the scrotum and testis and is known as the tunica vaginalis. Occasionally, the testes in the human male do not descend into the scrotal sac; this condition of nondescent, which is known as cryptorchidism, may result in sterility if not corrected by surgery or the administration of hormones. Retention of the testes within the body cavity subjects the germ cells to temperatures that are too high for their normal development; the descent of the testes into the scrotum in higher animals keeps the testes at optimum temperatures.

Respiratory Systems in Other Animals

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The need to take in oxygen and expel carbon dioxide is almost universal among organisms. The movement of these gases between an organism and its environment, called gas exchange, is accomplished in a variety of ways by different organisms. In one-celled aquatic organisms, such as protozoans, and in seaweeds, sponges, jellyfish, and other aquatic organisms that are only a few cell layers thick, oxygen and carbon dioxide diffuse directly between the water and cells. Diffusion works for these simple organisms because all cells of the organism are within a few millimeters of an oxygen source.

Animals with many cell layers cannot rely on diffusion because cells several layers deep in the body would die before oxygen reached them. As a result, for gas exchange, more-complex animals require special respiratory organs, such as gills or lungs, in combination with circulatory structures, such as blood, blood vessels, and a heart. The earliest development of these gas exchange structures is seen in roundworms, microscopic invertebrates abundant in water and moist soil. In roundworms, oxygen diffuses through the skin into a fluid that fills an internal cavity. As the worm moves, the fluid sloshes around in the cavity, bringing oxygen into contact with the digestive system, reproductive organs, and other structures in the cavity. This primitive circulatory system is called an open circulatory system because the fluid is not contained within vessels. In clams an open circulatory system is combined with a heart that pumps fluid around the internal cavity. Clams also use gills, thin-walled filaments that are extensions of the body surface. Gills provide a more extensive surface area for gas exchange than the body surface alone, enabling clams and larger organisms to obtain the amount of oxygen they need. Fish have gills, a heart, and a closed circulatory system, one in which blood is transported in vessels by the pumping action of the heart.

Relatively simple land-dwelling organisms, including some plants, fungi, and animals such as flatworms, accomplish gas exchange by diffusion. More-complex organisms, however, rely on specialized respiratory structures. Instead of gills, whose delicate filaments collapse unless supported by water, land animals use lungs. Located inside the body, lungs are formed by the infolding of membranes. The folds form a single balloon-like sac, as in amphibians; they may be arranged in stacks, as in the book lungs of spiders; or they are composed of millions of tiny air sacs, such as the lungs of most mammals. In virtually all vertebrates, a heart and a closed circulatory system work with the lungs to deliver oxygen and to remove carbon dioxide from cells.

Insects have a unique respiratory system made up of small tubes called tracheae. The tracheae connect all parts of the body to small openings on the surface of the insect. Oxygen and carbon dioxide are transported through the tracheae, and from the tracheae to the blood of the insect by diffusion. The blood of most insects is contained in an open circulatory system and is moved around the internal organs by a heart.

The respiratory system of birds, adapted for flight, is very different from that of land-bound animals. The lungs have two openings, one for taking in oxygen-filled air; the other for expelling carbon dioxide-laden air. Rather than ending up in alveoli, the air loops through the lungs so that the oxygen flow through the lungs is continuous. This design enables birds to obtain the amount of oxygen they need to power the extremely high energy demands of flight.

The Vertebrate Nervous System

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Vertebrate Brains
Although all vertebrate brains share the same basic three-part structure, the development of their constituent parts varies across the evolutionary scale. In fish, the cerebrum is dwarfed by the rest of the brain and serves mostly to process input from the senses. In reptiles and amphibians, the cerebrum is proportionally larger and begins to connect and form conclusions about this input. Birds have well-developed optic lobes, making the cerebrum even larger. Among mammals, the cerebrum dominates the brain. It is most developed among primates, in whom cognitive ability is the highest.

Vertebrate animals have a bony spine and skull in which the central part of the nervous system is housed; the peripheral part extends throughout the remainder of the body. That part of the nervous system located in the skull is referred to as the brain; that found in the spine is called the spinal cord. The brain and the spinal cord are continuous through an opening in the base of the skull; both are also in contact with other parts of the body through the nerves. The distinction made between the central nervous system and the peripheral nervous system is based on the different locations of the two intimately related parts of a single system. Some of the processes of the cell bodies conduct sense impressions and others conduct muscle responses, called reflexes, such as those caused by pain (see Reflex).

In the skin are cells of several types called receptors; each is especially sensitive to particular stimuli. Free nerve endings are sensitive to pain and are directly activated. The neurons so activated send impulses into the central nervous system and have junctions with other cells that have axons extending back into the periphery. Impulses are carried from processes of these cells to motor endings within the muscles. These neuromuscular endings excite the muscles, resulting in muscular contraction and appropriate movement. The pathway taken by the nerve impulse in mediating this simple response is in the form of a two-neuron arc that begins and ends in the periphery. Many of the actions of the nervous system can be explained on the basis of such reflex arcs, which are chains of interconnected nerve cells, stimulated at one end and capable of bringing about movement or glandular secretion at the other.

Digestive Systems of Animals

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The simplest invertebrates (animals without backbones) do not have specialized digestive organs. Single-celled organisms, such as amoebas, rely on intracellular digestion (digestion within the cell). Some many-celled organisms, such as the sponge, also use intracellular digestion. The sponge obtains the tiny organic particles that make up its diet from water passing through its body. Water enters through the sponge’s pores and leaves through an opening called the osculum. As water flows through the interior canals of the sponge, specialized cells that line these canals, called collar cells, catch and engulf organic matter. Inside the collar cells, sacs called vacuoles form around the food and enzymes digest it. The digested food then passes to other cells in the sponge’s body.

Intracellular digestion meets the needs of simple animals, but more complex organisms require systems that are more specialized. Animals such as jellyfish and nonparasitic flatworms combine the intracellular process with some specialized digestive organs. These animals have a definite mouth and a saclike cavity, which is lined with digestive cells that secrete enzymes. Digestion begins when the enzymes break down food inside the cavity in an extracellular (outside the cell) process. Cells then engulf the partly digested food, and an intracellular process similar to that of sponges completes digestion. Wastes are excreted through the mouth.

Most of the more complex invertebrates and all vertebrates (animals with a backbone) digest food entirely through extracellular processes. Food moves in one direction, from mouth to anus, through the series of organs that make up the alimentary canal. Specialization of various parts of the alimentary canal improves the body’s ability to break down food and absorb various kinds of nutrients. The mouth of many animals contains teeth or other structures to break up large lumps of food. Behind the mouth, the pharynx and esophagus swallow the food and move it to the stomach. The stomach temporarily stores the food, mixes it with digestive juices, and carries out some digestion.

Digestion is completed in the intestine. The liver and pancreas pour their digestive juices into the anterior (front) end of this organ. After the anterior intestine absorbs the usable products of digestion, the walls of the posterior (rear) intestine absorb leftover water. In vertebrates the anterior intestine is called the small intestine; the posterior intestine is the large intestine. Feces, composed of unabsorbed and indigestible food residues, form in the posterior intestine, where they are stored until they are excreted through the anus.

Within this basic plan, the specific components of the digestive system vary enormously from one animal to another. For example, a fish’s pharynx contains gill slits for breathing but has no digestive function. An earthworm’s stomach consists of two organs: a crop, in which food is stored, and a muscular gizzard, which carries out mechanical digestion by grinding food against particles of sand. The stomachs of ruminant mammals, such as cattle and deer, consist of three or four compartments, each performing a specific function. Amphibians, reptiles, and birds have an organ called a cloaca, which serves as an exit for both digestive wastes and sex cells.

Circulatory Systems in Non-Humans

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One-celled organisms and many simple multicelled animals, such as sponges, jellyfishes, sea anemones, flatworms, and roundworms, do not have a circulatory system. All of their cells are able to absorb nutrients, exchange gases, and expel wastes through direct contact with either the outside or with a central cavity that serves as a digestive tract.

More complex invertebrates have a wide range of circulatory system designs. These invertebrate circulatory systems are classified as either open or closed. Open systems—found in starfishes, clams, oysters, snails, crabs, insects, spiders, and centipedes—lack capillaries, and the blood bathes the tissues directly. In closed systems, the blood is confined to a system of blood vessels. Invertebrates with closed systems include segmented worms, squids, and octopuses.

All vertebrate animals have closed circulatory systems. These systems are classified by the number of chambers in the heart, which determines the basic configuration of blood flow. Fish have two-chambered hearts with one atrium and one ventricle. Blood pumped from the ventricle travels through arteries to the gills, where it diverges into capillaries and exchanges gases. Leaving the gills, the capillaries reconvene into blood vessels that carry the oxygenated blood to the rest of the body, where the vessels again diverge into capillaries before reconvening into veins that return to the heart. In this way, the blood passes through first the respiratory organs (the gills) and then the systemic circulation between each pass through the heart.

Frogs and amphibians have three-chambered hearts, with two atriums and one ventricle. Blood pumped from the ventricle enters a forked artery. One fork, the pulmonary circulation, leads to the lung. The other fork, the systemic circulation, leads to the rest of the body. Blood returning from the pulmonary circulation enters the left atrium, while blood from the systemic circulation enters the right atrium. Although there is some mixing of oxygenated and deoxygenated blood in the ventricle, a ridge within the ventricle assures that most of the oxygenated blood is diverted to the systemic circulation and most of the deoxygenated blood goes to the pulmonary circulation. In reptiles, this ridge is more developed, forming a partial wall. In crocodiles, the wall is complete, forming a four-chambered heart like that found in mammals and birds.

Aquaculture

Aquaculture, farming of aquatic organisms in fresh, brackish or salt water. A wide variety of aquatic organisms are produced through aquaculture, including fishes, crustaceans, mollusks, algae, and aquatic plants. Unlike capture fisheries, aquaculture requires deliberate human intervention in the organisms' productivity and results in yields that exceed those from the natural environment alone. Stocking water with seed (juvenile organisms), fertilizing the water, feeding the organisms, and maintaining water quality are common examples of such intervention.

Most aquacultural crops are destined for human consumption. However, aquaculture also produces bait fishes, ornamental or aquarium fishes, aquatic animals used to augment natural populations for capture and sport fisheries (see Fishing), algae used for chemical extraction, and pearl oysters and mussels, among others.

Aquaculture is considered an agricultural activity, despite the many differences between aquaculture and terrestrial agriculture. Aquaculture mainly produces protein crops, while starchy staple crops are the primary products of terrestrial agriculture. In addition, terrestrial animal waste can be disposed of off-site, whereas in aquaculture such waste accumulates in the culture environment. Consequently, aquaculturists must carefully manage their production units to ensure that water quality does not deteriorate and become stressful to the culture organisms.

Ruminant

Ruminant, even-toed animal that regurgitates and masticates its food after swallowing. The majority of ruminants have four nipples; they usually have sweat glands only on the muzzle and between the toes. Most species bear horns that may be permanent or may be shed periodically. The division includes three subdivisions: Tragulina, containing the chevrotains and characterized by a stomach with three chambers; Tylopoda, consisting of the camel, dromedary, llama, alpaca, guanaco, and vicuña, and characterized by a stomach with three distinct chambers; and Pecora, containing all sheep, goats, antelope, deer, gazelles, giraffes, and domestic cattle, and characterized by the presence of a distinct four-chambered stomach. Pecoran animals are known as true ruminants. Between the esophagus and the intestine, the stomach chambers of a true ruminant are the rumen, the reticulum, the omasum, and the abomasum, or rennet bag.

Scientific classification: Ruminants make up the suborder Ruminantia of the order Artiodactyla.

Amoeba

Amoeba, any of a group of unicellular organisms characterized by their locomotive method of extending cytoplasm outward to form pseudopodia (false feet). The amoeboid group includes hundreds of different organisms, ranging in size from about .25 to 2.5 mm (about 0.0098 to 0.098 in). Amoebas are considered the most primitive animals and are classified in the kingdom Protista. All amoeboid organisms have thin cell membranes, a semirigid layer of ectoplasm, a granular, jellylike endoplasm, and an oval nucleus. Some species live on aquatic plants and some in moist ground; others are parasitic in animals.

Amoebas also use pseudopodia for feeding. Chemical stimuli from smaller organisms, the amoeba's food, induce the formation of pseudopodia, pairs of which envelop the organism, at the same time forming a cavity, or vacuole. A digestive enzyme secreted into the cavity breaks down this food into soluble chemical substances that then diffuse from the cavity into the cytoplasm. Undigested food and wastes are excreted through the ectoplasm, which also absorbs oxygen from the surrounding water and eliminates carbon dioxide, a by-product of metabolism, in a form of respiration. After a period of growth, the amoeba reproduces by splitting into two equal parts.

At least six forms of amoeba are parasitic in humans. Most important of these is Entamoeba histolytica, which causes amebiasis and dysentery. The diseases often occur in epidemics when raw sewage contaminates water supplies or when soil is fertilized with untreated human wastes.

Sipuncula

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Peanut Worm, common name for small, unsegmented marine worms having bulb-shaped bodies and bearing crowns of tentacles on long, slender, processes, called introverts, that can turn inside out. Peanut worms are common but inconspicuous, frequently nestling among shells; they can burrow, but they move slowly. The main part of the body contains a single cavity (the coelom) filled with fluid under pressure from the body musculature. When the introvert turns inside out, the tentacles at its tip are used for feeding on small particles of organic matter. The placement of peanut worms in the animal kingdom is uncertain, but they are probably close to the annelid worms. Little structural diversity exists within the group; about 250 species have been described.

Scientific classification: Peanut worms make up the phylum Sipuncula.

Rotifera

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Rotifer, any of a phylum of multicellular, generally microscopic, aquatic animals that are abundant worldwide, and are most frequently found in freshwater bogs, ponds, and puddles. Rotifers vary in shape but always have retractable, hairlike crowns of cilia that, in motion, resemble turning wheels. (Among the first microscopic life forms to be studied, they were commonly known as wheel animalcules.) The animals can attach themselves temporarily to surfaces by means of a cementing secretion from the “foot” of the body. They reproduce sexually, but males are rare; except under severe conditions, the eggs develop parthenogenetically. Rotifers feed on other microorganisms; a few species are parasitic.

Scientific classification: Rotifers make up the phylum Rotifera.

Porifera

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Sponge, any of several thousand species constituting a phylum of simple invertebrate animals. Sponges are mainly marine, with a few freshwater species. They are abundant throughout the world and especially in tropical waters, where they and other invertebrates such as corals are important in the formation of calcareous deposits.

The structural components of a sponge include the outer, protective layer of cells and the spiny spicules, which form the skeleton. Sponges feed off microorganisms in the water that flow in through small openings known as ostia. The flagella on the inner layer of cells move the water through the sponge, absorbing food particles as the water flows past. Eventually the water exits through the osculum, the cavity at the top of the sponge.

Scientific classification: Sponges make up the phylum Porifera. Four major groups exist. All the Calcarea are marine, with skeletal spicules composed of calcium carbonate. The Hexactinellida are found in the deep sea; because their skeleton is made of silica in beautiful six-pointed arrangements, they are called glass sponges. The Demospongiae (95 percent of all living species) include the few freshwater forms. Their skeletal network is made of spongin, a rather flexible protein material (that of a bath sponge made from a real sponge), and in some species silica spicules are also present. The Demospongiae include the carnivorous Mediterranean sponge, which is classified as Asbestopluma hypogea. The Sclerospongiae have a combination of a thin silica and spongin skeleton that surrounds a larger, central calcareous skeleton.

Pogonophora

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Pogonophore, also beardworm, any member of a phylum of deep-ocean animals resembling worms that feed by means of long, hollow tentacles, each with a double row of hair-like cilia. Pogonophores live in tubes they secrete in ocean-bottom oozes, sometimes near hydrothermal vents. The animals may be more than 30 cm (more than 1 ft) long but are never more than about 2 mm (about 0.08 in) in diameter. The body has no digestive tract, and food is absorbed directly through a front region bearing up to 250 tentacles. The sexes are separate. Pogonophores were first discovered in Indonesia in 1900. Their relationship to other phyla is not yet certain, although they are clearly related to acorn worms.

Scientific classification: Pogonophores make up the phylum Pogonophora.

Platyhelminthes

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Flatworm, common name for soft-bodied, usually parasitic animals, the simplest of animals possessing heads. They are bilaterally symmetrical and somewhat flattened, and most are elongated. Three main classes are included in the flatworm phylum: tapeworms, which in the adult stage are parasitic in the digestive tracts of animals; flukes, which are parasitic in various parts of different animals; and planarians, which are free-living and nonparasitic. Some authorities include a group of unsegmented marine worms. Other authorities consider them a separate phylum.

The ectoderm (outer surface) of the free-living flatworms is usually covered with cilia; in the parasitic forms the ectoderm usually secretes a hardened material called cuticle. A well-developed musculature, found directly under the epidermis (skin layer), allows the body to expand and contract, thus changing the body shape to a remarkable degree. Vivid pigmentation is sometimes present in the free-living forms, but the parasitic forms are usually unpigmented. Flatworms have no true body cavity; the spaces between the organs are filled with a compact connective tissue called parenchyma. Except in the simplest forms, one end of the body is more specialized for sensory perception, and locomotion takes place in the direction of specialization. The oral and genital openings are on the ventral (under) side. When present, the digestive tract is either saclike or branched and has only one opening. This opening may be equipped with a sucker, as in the flukes, or, as in most planarians, it may have a well-developed pharynx. The nervous system consists of a network with a large ganglion (brain) and various longitudinal nerve cords forming the principal parts. Sensory cilia and “eye spots” may be present in the free-living forms and in the larvae of the parasitic forms. The flatworm has no blood or vascular system. Specialized cells possessing cilia, called flame cells, lead from the interior to one or more openings in the exterior by means of a network of tubes. Together these structures form the excretory system. The reproductive system is highly complex and occupies a large portion of the interior of the animal. Although flatworms are almost all hermaphroditic (both male and female reproductive organs are present in each individual), the eggs and sperm are formed separately. These germ cells either leave the body by separate openings or enter a common chamber, called the genital atrium. Flatworms also are able to reproduce asexually both by binary fission—that is, by pinching themselves apart to become two—and by regeneration, producing an entire new worm from a piece that has been cut off.

Free-living flatworms are found in almost every kind of environment, on land and in fresh and salt water. These forms feed mainly on plankton. The parasitic flatworms often display a complicated life cycle, which may require development in four or five hosts before completion.

Scientific classification: Flatworms constitute the phylum Platyhelminthes. Tapeworms constitute the class Cestoda. Flukes constitute the class Trematoda. Planarians constitute the class Turbellaria.

Phoronida

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Phoronids, common name for a wormlike marine invertebrate animal, a kind of tube worm. There are about 12 species of phoronids. They are sedentary animals that can be as long as 38 cm (15 in). Most species secrete a protective tube in the mud or sand bottoms of shallow seas.

The phoronids are regarded as closely related to the bryozoans and brachiopods because they possess a lophophore, a horseshoe-shaped structure carrying ciliated tentacles (see Tentaculata). The tentacles serve to catch food materials suspended in the water, and the cilia move mucus-entrapped food to the mouth, located at the center of the lophophore. The digestive tract is U-shaped, so the anus is located near the "head" of the animal but outside the lophophore.

The phoronids possess a well-developed circulatory system. Some species are hermaphroditic, in which one individual may have both male and female organs. Fertilization generally occurs outside the body; however, in some species, the eggs develop at the base of the lophophore tentacles.

Scientific classification: Phoronids are members of the phylum Phoronida, of the kingdom Animalia.

Pentastomida

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Tongueworm is any of a group of parasitic, wormlike animals found in the respiratory system of vertebrates, especially reptiles. Tongueworms cling to the lining of the nasal passageways and lungs where they feed on the blood of the host. Also called pentastomids, tongueworms occur worldwide, although most species are tropical.

The body of the tongueworm is highly modified for parasitism, that is, obtaining nourishment from the body of another, usually larger host organism (see Parasite). Adults of most species are 2 to 13 cm (0.8 to 5.1 in) long. Their mouth is jawless and sometimes projects outward as a trunklike proboscis. The mouth is flanked by two pairs of small, fingerlike projections equipped with claws that are used to attach to the host. The name pentastomid, meaning five-mouthed, derives from the fact that, in some species, these four grasping organs resemble the proboscis and look like additional mouths. The single, true mouth leads to a long, straight gut, the front of which forms a muscular pharynx for sucking blood. Frontal glands near the mouth of the tongueworm produce substances that break down the host tissue or prevent the host's blood from clotting. Like most internal parasites, tongueworms have no organs of excretion, gas exchange (breathing), or circulation. The tongueworm's blood flows freely throughout the body cavity. The tongueworm's nervous system consists of a frontal ganglion, or primitive brain, and a ventral nerve cord that runs along the bottom of the body cavity.

The sexes of tongueworms are separate, and the females are larger than the males. Fertilization is internal and occurs within the primary host. The larva has two or three pairs of walking legs, each with two claws. The larva may be autoinfective, that is, it remains in the primary host. If not autoinfective, the larva may form a protective capsule, called a cyst, which either moves through the digestive tract to be deposited with the host's feces or moves passively out of the host's nose or mouth.

Tongueworm eggs may pass out of the host before they hatch. If an intermediate host animal accidentally swallows the cyst or egg, the infective larva emerges inside the new host. It bores through the gut wall and completes its development into the next infective stage. If the intermediate host is eaten by a predator, the predator may acquire the pre-adult tongueworm. The juvenile tongueworm then crawls up the esophagus to the respiratory system and implants itself in the lungs or nasal passageways to complete the cycle. The larvae of one species of tongueworm that infests reindeer can infest the next generation of hosts by boring through the placenta. Examples of typical intermediate hosts and their corresponding primary hosts are cockroaches and lizards, fish and crocodiles, and rabbits and dogs.

Scientific classification: Tongueworms make up the phylum Pentastomida. Their relationship to other animals is not clear; they resemble both arthropods and annelids. One theory on the origin of this group is that they are actually highly modified crustaceans.

Onychophora

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Velvetworm, any of a group of soft-bodied, many-legged, worm-like animals known for their velvety cuticle, or outer covering, and their resemblance to both arthropods (insects, spiders, and crustaceans) and annelids (segmented or true worms). They live in moist tropical and temperate habitats south of the tropic of Cancer. Velvetworms are also known as onychophorans or peripatuses, after the most familiar genus. About 80 species of velvetworms are known.

Mature velvetworms range from 1.5 to 15 cm (0.6 to 5.9 in) in length and may be blue, orange, green, or black in color. Their thin, flexible cuticle, or outer skin, bears many small, scaly, wartlike tubercles and sensory hairs, which produce a velvety appearance. Velvetworms have 14 to 43 pairs of stubby, claw-tipped legs called lobopods. The lobopods contain no muscles. Velvetworms control their body and limb movements with hydrostatic pressure changes within their body cavity. Velvetworms use these pressure changes to lift and lower their legs in waves, as their body creeps along with slug-like motion.

When conditions are too dry or too wet, velvetworms hide in burrows or protected niches until conditions improve. They are nocturnal, appearing only at night, and prey on other small invertebrates. The velvetworm's jaws hold the prey while it is being eaten. Salivary glands produce enzymes that partly digest tissues so the meal can be sucked up. When disturbed or threatened, velvetworms shoot a sticky, quick-hardening slime from two oral papillae, wartlike bumps on either side of the mouth. They can shoot the slime up to 50 cm (19.7 in). The slime is produced by slime glands originating at the base of the oral papillae and may also be used to immobilize prey.

The velvetworm has a pair of fleshy, ringed antennae on its head and a pair of eyes near the bases of the antennae, each with a lens and retina. Blood is circulated by a tube-shaped dorsal heart (located near the top of the body cavity) and flows freely within the body cavity around the internal organs; it carries no oxygen. Gases are exchanged through a system of minute tracheae, or breathing tubes, which open into pits located throughout the cuticle. The pits cannot be closed and are a major source of water loss for the moisture-dependent velvetworm. The velvetworm takes in water by drinking and by absorption through special ballooning sacs on the lobopods. Wastes are excreted from paired sacs called coelomoducts or nephridia, which are present on most body segments and which lead to pores on the legs. The slime glands and female sexual ducts are modified coelomoducts. The brain gives rise to a pair of ventral nerve cords, which run along the bottom of the body cavity and are connected by ladderlike cross nerves.

The sexes are separate in velvetworms; males are generally smaller and may have fewer legs than females. In the species where sexual behavior is known, fertilization occurs through copulation.

Scientific classification: Velvetworms make up the phylum Onychophora. The most familiar species are in the genus Peripatus.

Nematoda

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Roundworm, also nematode, common name for any of a phylum of unsegmented terrestrial, freshwater, or marine worms. Roundworms are almost worldwide in distribution and are abundant in the surface layers of soils. Many of them are economically and medically harmful, living as parasites in plants and animals, including humans. Roundworm infections are common and frequently go unnoticed, but several species cause serious diseases.

Roundworms are cylindrical, tapering animals with simple bodies consisting of an interior gut and a muscular outer wall, separated by a fluid-filled cavity called a pseudocoel (see Animal: Coelom). The outer wall secretes an elastic cuticle that is molted four times during the animal's lifetime. Species range in size from microscopic to about 10 cm (about 4 in) long. Most species have separate sexes, but a few are hermaphroditic; fertilization is internal. The young roundworms, which resemble the adults, develop without metamorphosis.

Although numerous roundworms are free-living, the parasitic forms are of greatest economic interest. One important group, the ascaroid nematodes, includes the threadworms and the common worm of puppies. Another contains the eelworms, which produce root knot of cotton, and forms that produce earcockle of wheat. Other, medically significant forms of roundworm include the various genera known as hookworm; the filaria, which cause elephantiasis; the trichina worm, the cause of trichinosis; and the whipworm, which infests the human intestine.

Scientific classification: Roundworms make up the phylum Nematoda. Threadworms belong to the genus Ascaris. The common worm of puppies is classified as Toxocara canis. Eelworms belong to the genus Heterodera, the forms that produce earcockle of wheat belong to the genus Tylenchus, and the trichina worm belongs to the genus Trichinella. The human whipworm is classified as Trichuris trichiura.

Nematomorpha

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Horsehair Worm, member of a group of long, slender, unsegmented worms that resemble horsehair. The name of their phylum, Nematomorpha, means “threadlike.” Long ago, people thought that these worms were actually hairs from a horse's tail that had come alive. The immature worms, or larvae, are all parasites, feeding within the bodies of leeches and arthropods such as insects and crustaceans. The adults are free-living—that is, they do not depend on a host.

Horsehair worms are 1 to 3 mm (0.04 to 0.12 in) in diameter and up to 1 m (3.3 ft) long. There are two classes of horsehair worms. The first, known as nectonemes, live within the bodies of crabs and their relatives. The adults are planktonic, drifting in the currents of the open ocean. The body has two rows of bristles down each side, which aid in buoyancy. Nectonemes have only one gonad, or sex organ. The second class of horsehair worms is made up of the gordian worms, so named because they appear to tie themselves in knots, much like the complex Gordian knot of Greek mythology. The larvae parasitize leeches and arthropods such as grasshoppers, crickets, beetles, millipedes, and centipedes. The adults occur worldwide in freshwater or moist terrestrial habitats. Gordian worms possess a pair of gonads.

The body of the adult horsehair worm is covered by a relatively thick outer cuticle, or skinlike covering. Underlying the cuticle is a layer of longitudinal muscle running the length of the body. The horsehair worm has no circular muscle, and thus its movement is limited to lashing and curling. The digestive system is simple, even in the free-living adults. There are no excretory, circulatory, or respiratory organs. The larvae probably take up nutrients directly through the body wall during the parasitic phase. Feeding has not been observed in the adults, and most have no mouth, but they grow considerably after they leave the host at the end of the larval stage. It is therefore likely that they do feed by some means. The nervous system consists of a nerve bundle at the head end and a nerve cord running down length of the body. Around the head of some species is a ring of pigmented tissue that may be light-sensitive, but horsehair worms have no eyes. The outer layer of the cuticle is often covered with bumps, some of which may detect touch and some of which produce a lubricant.

Scientific classification: Horsehair worms make up the phylum Nematomorpha. The nectonemes make up the class Nectonematoidea, which has only one genus, Nectonema. The gordians make up the class Gordioidea.

Mollusca

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Mollusk is a common name for members of a phylum of soft-bodied animals (Latin mollus, “soft”), usually with a hard external shell. The mollusks represent a diverse group of marine, freshwater, and terrestrial invertebrates, including such varied forms as snails, chitons, limpets, clams, mussels, oysters, octopuses, squid, cuttlefish, tusk shells, slugs, nudibranchs, and several highly modified deep-sea forms. They all have one anatomical feature in common, the presence of a shell at some stage in the life cycle. Although most mollusks have a shell as adults, the octopus, squid, and deep-sea forms do not. They do however have a small, shell-like structure, called a shell gland, present for a short time during embryonic development. The mollusk phylum is the second largest in the animal kingdom, after the arthropods.

Scientific classification: Mollusks make up the phylum Mollusca. In the class Aplacophora, the body is wormlike. No shell exists, only a tough mantle, and the foot has virtually been lost. The three orders of the class Polyplacophora (chitons) have a series of eight shell plates (valves) in a row and are well adapted to clinging on rocks. The mainly fossil Monoplacophora is now known to have one living genus, Neopilina, discovered in deep water in 1952. The animal has a single flat shell and multiple gills. The class Bivalvia have a shell divided into two valves, and they feed with their gills. As a consequence the head is poorly developed. Members of the class Scaphopoda (tusk shells) have a long, tapered, slightly curved shell and live on sandy bottoms. Members of the class Gastropoda (snails and slugs) are asymmetrical and have only one shell or, as in slugs, are shell-less. The three subclasses of the Gastropoda are the Prosobranchia (mostly marine snails, with three orders), Opisthobranchia (sea slugs and their allies, with eight orders), and Pulmonata (lunged mollusks, largely freshwater and terrestrial, with two orders). The class Cephalopoda are modified by reduction of the foot and shell and the development of arms around the mouth. The two subclasses are Nautiloidea ( Nautilus, with four gills and other archaic traits such as an external shell) and Coleoidea (octopuses, squid, and cuttlefish, with two gills and other advanced traits).

Mesozoa

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Mesozoa (Greek mesos, “middle”; zōion, “animal”), phylum or superphylum of life forms, sometimes thought to be transitional from unicellular to multicellular organisms. The body consists of a layer of outer cells surrounding internal reproductive cells; it contains no real organs. Except at the time of dispersal, mesozoans live as internal parasites of marine invertebrates. Some authorities consider them degenerate flatworms; others deny that they are animals. The group contains about 50 species placed in two classes or orders.

Loricifera

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Loriciferan, name of a group of tiny marine animals, first identified in 1974, and established as a new phylum in 1983 by Danish zoologist Reinhardt Kristensen. Loriciferans are unusual, nearly microscopic creatures 0.25 to 0.33 mm (0.01 to 0.013 in) long. They are generally oval in shape. The head ends in a beaklike conical mouth that can be retracted into the body. The mouth is surrounded by nine rings of bristly scales, the first set of which point forward, the rest backward. The body is encased in several hard plates that resemble a corset, from which the group gets the name Loricifera, meaning “corset-bearer.” At the hind end of the body is an anus. Much of the body cavity of adults is filled with either developing eggs or testes. There is a specialized immature form known as a Higgins larva. It has a pair of oarlike scales, called toes, at its hind end which are used for locomotion. Little is known about the reproduction and life history of loriciferans.

Scientific classification: Loriciferans make up the phylum Loricifera. The first discovered loriciferan is classified as Pliciloricus enigmatus.

Cycliophora

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Symbion, a tiny marine organism so different from any other that it has given rise to a new phylum (see Classification). Symbion (meaning “living with”) is a commensal, a harmless companion that lives on the mouth hairs of the Norwegian lobster and feeds on the scraps the lobster leaves behind after its messy meals. Symbion's distribution is unknown but may coincide with that of its lobster host.

Symbion takes on different forms throughout its strange life cycle. The most common form is the feeding stage, which is about 0.1 mm (0.004 in) long. The body is urn shaped and attached to the lobster by a short stalk and an adhesive disk. On the other end is a funnel-like mouth topped with a ring of microscopic hairs, or cilia. The mouth leads to an S-shaped esophagus, or throat, and a U-shaped digestive system. The first part of the U forms a stomach lined with ciliated and secretory cells; farther along it becomes an intestine, also lined with cilia. Because of the U shape of the digestive system, the anus, or excretory opening, is near the mouth. The two-lobed brain is situated between the funnel base and the anus. The outer surface of the animal is layered and sculptured with pentagonal shapes. Symbion replaces its mouthparts and nervous system several times during its life by forming an internal bud consisting of a new digestive tract and brain. As this bud matures, it eventually replaces the old structures.

The reproductive cycle is unusual and complicated and is an important reason for putting Symbion in its own phylum. Symbion reproduces in two different phases: asexual budding and sexual fertilization. In the asexual phase, the feeding stage forms an internal bud containing an embryo that develops without fertilization into a Pandora larva, a type of free-swimming larva. The Pandora larva escapes and settles on the same lobster host, developing into another feeding stage.

Symbion's discovery was announced by the Danish scientists Peter Funch and Reinhardt Mobjerg Kristensen in 1995. They documented its curious life cycle and provided evidence to distinguish it from other species to which it may be related, such as rotifers, moss animals, and entoprocts, small marine and freshwater animals that Symbion resembles.

Scientific classification:Symbion pandora is the sole member of the phylum Cycliophora, or “wheelmouth” animals. It is also the only member of the family Symbiidae, order Symbiida, and class Eucycliophora.

Hemichordata

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Acorn Worm, common name for simple, wormlike marine animals in the hemichordate phylum. They are of special interest because of their close relationship to chordates. This connection is evident in the adult anatomy. Some representative acorn worms have gill slits, traces of a supporting structure resembling a notochord, and a tubular nerve cord, which are features characteristic of vertebrates. The larval stages of acorn worms, however, are very much like those of echinoderms such as starfish, indicating a remote common ancestry of echinoderms and vertebrates.

The hemichordates are divided into two classes comprising about 50 species. The first class, the acorn worms, consists of animals that average 10 cm (4 in) in length, although some species may be up to 1.5 m (up to 5 ft) long. They construct burrows, commonly U-shaped, in sand of shallow seafloors using an extendable, muscular proboscis attached to a thick collar that resembles an acorn—hence the name. They secrete a slime that collects food particles on the proboscis and collar, but some species filter sediments and sand through a complicated pharynx with many gill slits. The second class consists of small, usually colonial animals of the deep sea. They are not worm-shaped but stout, and they usually construct tubes. Food is captured by tentacles that project from the tube. The body is much simplified, and gill slits are reduced to one pair or none.

See also Balanoglossus.

Scientific classification: Acorn worms make up the class Enteropneusta in the phylum Hemichordata. The other class of hemichordates is Pterobranchia.

Gnathostomulida

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Gnathostomulid, group of microscopic marine worms that live between grains of sand in shallow ocean water. These invertebrates are able to live for long periods without oxygen (see Anaerobe) and are common in stagnant bottom muck and black sand. More than 80 species of gnathostomulids have been described.

Gnathostomulids resemble flatworms and were once classified in the same phylum. Like flatworms, gnathostomulids are bilaterally symmetrical (both sides of the body are identical along the midline) and have waving body cilia (tiny hairlike projections); they lack an anus and a coelom (internal body cavity) and are hermaphroditic (individuals have both male and female sex organs). Unlike flatworms, however, each epithelial (skin) cell in gnathostomulids has only one hairlike cilium. In addition there are significant differences in sperm structure between gnathostomulids and flatworms.

Gnathostomulids are generally 0.5 to 1.0 mm (0.02 to 0.04 in) in length with a slight constriction of the body separating the head from the trunk. They move their long transparent bodies with the propelling force of their rotating cilia. Unlike flatworms, they have the ability to reverse the direction of the ciliary beat. The contractions of three or four pairs of longitudinal muscle fibers also contribute to movement. Gnathostomulids have a mouth located on the underside of the body behind the head. They graze on bacteria and fungi using a pair of toothed jaws that grasp and push food into the intestinal sac.

Even though gnathostomulids possess both male and female sex organs, they cannot fertilize themselves. Eggs are fertilized when one gnathostomulid injects a packet of sperm into another individual’s body using a penislike organ that is equipped with a piercing tip in some species. A single large egg is released by breaking through the body wall and develops without larval stages into an adult. The parent worm regenerates rapidly after releasing the egg. Some gnathostomulids appear to alternate between a nonsexual feeding phase and a sexual phase during which they do not feed.

Scientific classification: Gnathostomulids make up the phylum Gnathostomulida, and are members of the superphylum Aschelminthes.

Gastrotricha

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Gastrotrich, group of microscopic wormlike animals that are common in aquatic environments worldwide. The gastrotrich lives in bottom sediments and on the surface of submerged vegetation and debris. Its head is divided into lobes and its body is generally flat, transparent, and unsegmented. Its back and sides are spiny, bristly, or scaly. The flattened bottom surface bears small hairs, or cilia, in characteristic patterns that are helpful in classifying species. Along the sides of the body are adhesive tubes that are used for clinging to vegetation.

Gastrotrichs range in length from 0.05 to 4.0 mm (0.002 to 0.16 in). Marine gastrotrichs, those living in the ocean, are abundant on the surfaces of corals and in shallow sandy areas, while freshwater species prefer standing water such as puddles, marshes, and wet bogs. They may be present in habitats where there is much decay and they can withstand low levels of dissolved oxygen for short periods. The gastrotrich feeds as it moves, ingesting bacteria, algae, small protozoans, and organic debris from the substrate. Waving cilia on the head create currents that push food particles to the mouth.

Marine gastrotrichs are hermaphroditic (having both male and female sex organs) and individuals alternately produce eggs and sperm. Most freshwater species are entirely female and reproduce through parthenogenesis (egg development without fertilization). A female will produce one to five very large eggs in its lifetime. The eggs are of two types: thin-walled eggs that develop as soon as they are laid, and thick-walled eggs that require exposure to harsh conditions such as drying, freezing, or high temperatures before they develop. Gastrotrichs have no larval stages. On emergence from the egg, the gastrotrich is already about 25 percent of its adult size. It grows rapidly and some species reach sexual maturity in only three days. The lifespans of gastrotrichs in nature are unknown, but individuals in laboratory cultures live 3 to 22 days. Gastrotrichs are important as food for many organisms in aquatic food webs.

Scientific classification: Gastrotrichs make up the phylum Gastrotricha, which contains two orders. Animals in the order Macrodasyida are strictly marine, and those in the order Chaetonotida are primarily freshwater.

Entoprocta

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Entoprocts, tiny, mosslike invertebrate animals found in shallow waters. Entoprocts have tentacles and oval-shaped bodies, and usually live in branching colonies. Entoprocts lack a true coelom, or body cavity. A slender stalk attaches the body mass, or calyx, to either an inanimate object or another animal. The digestive tract is U-shaped. On the uppermost surface of the calyx are tentacles made of cilia arranged in a circle around the mouth and anus. The movements of the hairlike cilia create currents that sweep tiny animals and suspended particles into the mouth.

Entoprocts may live singly but are more often found as colonial groups, with many individuals arising from a single animal through the asexual process of budding. When sexual reproduction occurs, large yolky eggs are brooded in a special cavity within the circle of tentacles. With the exception of one freshwater genus, the entoprocts are exclusively marine. Because of their small size they are difficult to observe without the aid of a microscope.

Scientific classification: Entoprocts make up the phylum Entoprocta.

Ectoprocta

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Moss Animals, common name for two phyla of small, simple aquatic animals that feed with a crown of tentacles called a lophophore and usually form attached, mossy colonies. The classification of these two phyla has varied to reflect changing opinions about the relationship of moss animals to other phyla. Authorities who think the two groups have a close common ancestor retain the phylum name Bryozoa for all moss animals and treat endoprocts and ectoprocts as classes. Others use the term Bryozoa only for ectoprocts, and still others think that ectoprocts are related to the Tentaculata and endoprocts to the Aschelminthes.

Endoprocts, which are marine except for one freshwater species, have a globular body that is mounted on a stalk. The lophophore surrounds both mouth and anus. The animals reproduce both sexually and asexually, often forming colonies of connected individuals by the latter process. In ectoprocts, which are primarily marine, the lophophore does not surround the anus. The colonies that ectoprocts form by asexual reproduction are of varied structure, and each member usually has a hard, protective coating.

Scientific classification: Endoprocts make up the division Endoprocta, or Entoprocta. Ectoprocts make up the phylum Ectoprocta, or Bryozoa.

Echiura

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Spoonworm, common name for any of a small phylum of unsegmented worms with an internal body cavity and characterized by a stout body, or trunk, and a long flexible, spoon-shaped proboscis, or feeding organ. They are found on the ocean bottom from the shallow intertidal zone to depths of 10,000 m (32,808 ft.)

Mature spoonworms, also called echiurans, are sedentary bottom feeders that burrow in mud, sand, or debris. A few live in rock crevices or enclosures such as abandoned sand dollar shells. Most spoonworms are dull in color, but a few are green, red, or transparent. With the proboscis retracted, they range in length from 1 to 20 cm (0.4 to 7.9 in). In most species, the trunk of the spoonworm lies buried in the muck or debris on the ocean bottom while the proboscis extends outward to grope for food. The touch- and taste-sensitive proboscis is flexible and may extend up to 2 m (6.56 ft). It has a groove lined with cilia (tiny hairs) along its length. Glands on the proboscis secrete sticky mucus that picks up the debris on which the spoonworm feeds. The cilia in the groove wave to transport mucus and debris to the mouth. The skin of the spoonworm is rich with sensory cells and mucous glands. Three layers of muscle line the interior of the body cavity, or coelom. The spoonworm moves by exerting pressure with these muscles on the coelomic fluid.

Spoonworms have a three-part digestive system consisting of a foregut, a stomach, and a long, coiled midgut, in which most digestion takes place. The circulatory system is very simple, consisting of two vessels that run the length of the body cavity and does not include a heart. A ventral nerve cord along the underside of the body cavity and a ring of nervous tissue near the head comprise most of the spoonworm's nervous system. Spoonworms breathe by diffusion of oxygen through the skin. All spoonworms have a pair of funnel organs, pocket-like sacs with openings on the underside of the animal.

The sexes are separate in spoonworms and fertilization occurs externally in most species. Gametes, or sex cells, are produced in sex organs near the ventral nerve cord. Sperm or eggs usually exit the body through the funnel organs. In some species the eggs are retained and the organs function as a uterus. The larvae, classified as trochophores, are similar to those of polychaetes, or segmented worms, and in most species of spoonworm, they float freely in the ocean for several months before settling as adults. In some spoonworms, males and females are very different and sex is determined by an unusual mechanism. In the green bonellid, if the larva settles away from others of its species, it becomes a female, up to 8 cm (3.15 in) long. If, however, it lands on or near the proboscis of a female of the same species, the larva will become a male, due to a hormone produced by the female's proboscis. The tiny males are only 2 to 3 mm (0.08 to 0.12 in) long and live on the female's body or internally in one of her funnel organs.

Scientific classification: The spoonworms comprise the phylum Echiura. The green bonellid is Bonellia viridis. The innkeeper is Urechis caupo.

Echinodermata

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Echinoderm, common name for about 6000 living species constituting a phylum of marine animals, such as starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers. They usually show a superficial five-part radial symmetry, and generally are equipped with peculiar tube feet. The phylum name is derived from the spiny skin.

An echinoderm such as the starfish typically has a mouth surrounded by five arms that bear minute, fleshy tube feet with which the animal clings and crawls. The tube feet are supported by a complex internal hydraulic system that is inflated with seawater. Some echinoderms, especially brittle stars, crawl or swim by moving their arms. Often, as in sea urchins, rows of tube feet extend along the body surface, and arms are absent. The spines are particularly well developed in sea urchins. The skeleton, made up of calcium carbonate, may form a large proportion of the body, or, as in some sea cucumbers, it may be greatly reduced. The fossil record shows that the five-rayed pattern is a late evolutionary acquisition, and deviations from it are common. The animals have a well-developed gut, but the nervous and circulatory systems are simple. Echinoderms are slow-moving and without complex behavior patterns.

Echinoderms are common on the ocean bottom at all depths; in the deep sea they often make up the bulk of living material. They may be grazers (most sea urchins), feeders on small particles (many brittle stars), or predators (most starfish). Starfish and a few others are pests, as when found in oyster beds. Sea cucumbers and sea urchins are eaten by humans.

Scientific classification: Echinoderms make up the phylum Echinodermata. The living echinoderms are commonly divided into two subphyla. The stalked or attached Pelmatozoa consist of the single class Crinoidea, or sea lilies and feather stars, with 650 species. The free-living Eleutherozoa consist of the classes Holothuroidae, or sea cucumbers, with 900 species; Echinoidea, or sea urchins and sand dollars, with 850 species; Asteroidea, or starfish, with about 1500 species; and Ophiuroidea, or brittle stars, with about 2000 species. Many authorities, however, unite Asteroidea and Ophiuroidea into a single class, Asterozoa (or Stelleroidea). About 20 extinct classes are found in the fossil record.

Pictures of Invertebrates

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Gooseneck Barnacles
Gooseneck barnacles are crustaceans that live in the intertidal zones of temperate and cold water oceans, particularly in areas of heavy surf. The hard-shelled portion of the gooseneck barnacle’s body, which contains the internal organs, is supported on a long muscular stalk. Gooseneck barnacles use this stalk to latch onto floating objects or rocks, where they cluster in dense groups.





Gray Snakelocks Anemone
The gray snakelocks anemone, sometimes called the pink-tip, is related to the jellyfish, corals, and hydroids. Although sea anemones can move, they do not actively pursue prey but rather capture fish, shrimp, or other invertebrates that swim past or fall on them from above. The sea anemone’s color results from the presence of a symbiotic algae, called zooxanthellae, living in the anemone’s body tissues.





Great Gray Slug
All species of marine and terrestrial slugs are shelless mollusks. Terrestrial slugs, such as the great gray slug, generally feed on leaves and can cause considerable damage to trees and cultivated plants in gardens and greenhouses.







Green Hydra
The green hydra is the freshwater counterpart of the sea anemone, belonging to the same phylum as the sea anemone, coral, and jellyfish. Possessing six to ten tiny tentacles armed with potent stinging cells, the hydra captures small organisms from the surrounding water. It is often found in colonies of many individuals. The hydra is capable of regenerating lost body parts, even in cases where the injury has been extensive.




Hermit Crab
Hermit crabs are distinguished from other crabs by the lack of a shell on the abdomen. As a result, hermit crabs must find empty snail shells to temporarily inhabit. While walking or feeding, hermit crabs extend their antennae, claws, and two pairs of walking legs out of the shell opening. When threatened by predators, hermit crabs quickly withdraw their bodies back into the protection of the shell. The large claw, the last part of the body to be withdrawn, often acts as a door, closing off the interior of the shell from predators. As hermit crabs grow, they must continually find new, larger shells in which to live.


Human Head Louse
The human head louse, Pediculus humanus, is one of several kinds of lice with mouthparts specialized for sucking blood. The small, wingless insect has a flattened body about 3 mm long, with a claw on the end of each leg that helps it cling to the hair of its host. Females lay whitish eggs, called nits, once a day, attaching them to the hair with a sticky substance until they hatch in about a week. Head lice are unpleasant and undiscriminating guests. They infest people who bathe often as well as those who do not, leaving itchy red spots on their hosts’ scalps.


Millipede
The millipede is a segmented arthropod found in damp, moist habitats worldwide. The body of the millipede may have from 9 to over 100 individual segments, each of which bears a pair of legs. Millipedes feed on decaying vegetation and are generally harmless. When threatened or handled, they curl up in a tight ball. For defense against insect predators, millipedes rely on their stink glands, which secrete a noxious chemical substance that repels predators.



Murray River Crayfish
Although identical in shape to the marine lobster, the freshwater crayfish is more adaptable, being able to live in freshwater streams, ponds, and even terrestrial burrows. In some agricultural areas of Spain, crayfish are so abundant they are considered pests.






Portuguese Man-of-War
The Portuguese man-of-war is actually a colony of specialized polyps that all contribute to the well being of the colony by performing specific tasks, such as feeding or providing buoyancy control. The man-of-war has a relatively large, gas-filled bladder that provides flotation for the entire colony. A large collection of feeding polyps suspended from the underside of the float, each with a single, long tentacle, captures and digests food, which is then shared with the entire colony by means of interconnecting digestive cavities. Although the Portuguese man-of-war resembles a jellyfish, it belongs to Class Hydrozoa in the phylum Cnidaria, while true jellyfish belong to Class Scyphozoa in the same phylum.

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Pictures of Invertebrates

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Black Widow Spider
The female black widow spider, distinguished by the red hourglass marking on the underside of her abdomen, is probably the best known and most feared of all North American spiders. In spite of the female black widow’s small size of 1.2 cm (0.5 in.), its venom is quite toxic and may cause a wide range of symptoms in humans, including pain, swelling, nausea, and sometimes death. The male of this species is harmless to humans and lacks the hourglass marking on the abdomen.



Blue Spotted Sea Urchin
This underside view of the blue spotted sea urchin shows its mouth apparatus, commonly called Aristotle’s lantern. This structure is composed of five jaws arranged in a radial, or concentric, pattern. This arrangement allows the sea urchin to feed efficiently on a variety of marine algae and kelps.







Branching Coral Colonies
Branching coral is actually a colony of very small individual animals called coral polyps. Branching corals are considered hard corals, since they have a hard calcium carbonate skeleton. Their bright colors result from the presence of symbiotic algae that live in their body tissues and produce most of the food that the coral needs to survive.




Chambered Nautilus
A cutaway view of the shell of the chambered nautilus reveals the compartments that housed the nautilus when it was smaller. These smaller chambers, now connected together by a small calcified tube, regulate the buoyancy of the nautilus as it swims along. The compartments are filled with nitrogen gas, which is produced by the nautilus.






Clams
Clams have long been one of the most popular of the edible shellfish. Referred to as bivalve mollusks because of the two valves, or shells, that enclose the body, these small filter-feeding animals are commonly found in intertidal areas throughout the world. Strong internal muscles, a hinge ligament, and a calcified hinge at the apex of the shell allow the clam to protect itself against many types of predators by keeping the shell tightly closed. The prominent growth rings found on the outer surface of the clam shell are useful in determining the clam’s age.


Common Cuttlefish
Related to the octopus and squid, the common cuttlefish is an open water species of cephalopod mollusk that swims by undulating a continuous fin along the length of its short, fat body. The cuttlefish is commercially important throughout many parts of its range. A supporting rod of calcium carbonate present within the cuttlefish, called cuttlebone, is used commercially as a polishing agent and as a source of calcium and salts for captive birds and other animals.



Common Octopus
The common octopus has the typical octopus body form consisting of three main regions: the mantle, limbs, and head. The mantle, a large, muscular, bulbous sac, contains most of the internal organs, including the gills, digestive tract, ink sac, and reproductive organs. The limbs consist of eight arms. Along the length of each arm is a double row of suckers equipped with tactile and olfactory receptors.





Earthworm
Earthworms have a segmented, compartmentalized, cylindrical body and range in length from several centimeters (a few inches) to nearly 3.3 m (11 ft). They have no eyes, ears, or lungs. Earthworms breathe when air that is present between soil particles diffuses through their thin skins, and they are forced to the surface if these air pockets fill with rainwater. When a worm moves, it uses its longitudinal muscles to extend the front of its body into the soil ahead of it, pulling the back part up behind it. Setae, tiny projections from each segment of the worm, stick into the surrounding soil to keep the worm from slipping. The digging action of earthworms helps to aerate and mix the soil. Earthworms actually consume some of the soil as they dig, and their fecal deposits, called castings, also help enrich the soil.

Echinoderms
Members of the phylum Echinodermata, commonly called echinoderms, or spiny-skinned animals, are so named because of their spiny outer body coverings. Echinoderms differ from other animals in that they have a water vascular system that uses seawater to accomplish respiration, locomotion, and reproduction. The echinoderms include marine groups such as the sea stars (top, left), sea cucumbers (bottom, left), brittle or serpent stars (top, center), sand dollars (bottom, center), sea lilies (right), sea urchins, basket stars, heart urchins, and feather stars.

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Pictures of Insects

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Luna Moth
The full-grown luna moth has transparent circles bordered by light yellow and blue rings on its light green wings. Luna moths reproduce twice a year and eat the leaves of trees in the deciduous forests they inhabit. Now endangered because of contaminated food sources and pesticides, the luna moth is found only in North America.




Peacock Butterfly
The conspicuous eyespot markings of the peacock butterfly, Inachis io, make the insect unmistakable. The butterfly uses its patterning in self-defense, opening its wings if threatened by a bird and attempting to alarm the attacker by turning the pattern towards it. The butterfly also rubs its wings together to make a noise that helps scare off predators.





Praying Mantis
The praying mantis is so named for the prayerlike posture it assumes while waiting for its prey. Although the praying mantis generally eats insects and small tree frogs, the female will devour part of her own mate. Commonly found in tropical and warm temperate climates, the mantis was introduced into the United States to help control certain insect populations.







True Locust
The true locust is one of over 5000 species of grasshopper in the family Acrididae. Locusts travel in huge numbers capable of feeding on and destroying entire fields of cultivated plants and any nearby vegetation. Approaching swarms create an ominous hum and sometimes are large enough to block out sunlight.






Tumblebug
The tumblebug is one of 30,000 species of scarab beetles, which include dung beetles and chafer beetles. The tumblebug forages on decaying plant material and lays its eggs in small balls of dung, which then serve as a food source for the emerging larvae.



Walkingstick
Walkingsticks have the colors and shapes of the plants on which they live, resembling twigs, or dried leaves. The eggs of many species resemble seeds. Walkingsticks are vegetarians, feeding on leaves. Shown here is Extatosoma tiaratum, an Australian species resembling a spiny, leafy twig.





Worker Honey
As they fly from flower to flower, worker honey bees collect pollen grains and pack them onto their hind legs in special hair-fringed pockets known as pollen baskets (shown here holding a glob of yellow pollen on the hind leg). Nectar, the sweet liquid produced by flowers, is sucked into the honey stomach, an internal storage sac. In the hive, field bees deposit their pollen pellets into empty storage cells of the comb and regurgitate nectar to waiting hive bees. The hive bees mix some nectar with the pollen to make bee bread, a spoilage-proof larval food, and gradually concentrate the rest of the nectar into honey by dehydration.

Pictures of Insects

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American Cockroach
The American cockroach is one of the hardiest species of cockroaches. Unlike most other cockroach species, the American cockroach can fly and is also attracted to light.







Aphids
Aphids are small insects found throughout temperate regions of the world. They parasitize a variety of wild and commercially important plants by sucking out plant fluids. Because aphids exude a sweet, sticky fluid that can be used by certain species of ants, herds of aphids are often found guarded and tended by ants.





Blue Damselfly
After mating, blue damselflies deposit their eggs in water. The eggs hatch and develop into an aquatic larval form, which eventually leaves the water, undergoes metamorphosis, and changes into an adult.







Caterpillar
The larva of a butterfly or moth, known as a caterpillar, has a plump cylindrical body and chewing mouthparts. Most caterpillars eat leaves or other parts of plants and grow rapidly, shedding their skin several times as they grow. The caterpillar of a death’s head hawkmoth, Acherontia atropos, is pictured here.



Firefly
The firefly, belonging to the family Lampyridae, is one of a number of bioluminescent insects capable of producing a chemically created, cold light. Both males and females can generate the light, which is believed to attract them to each other. The light is produced when two chemical substances created by the firefly, luciferin and luciferase, come into contact with oxygen.





Forager Ant
A forager ant searches for food among blades of grass in South Africa’s Karoo National Park.






Fruit Fly Laying Egg
The small vinegar flies of the family Drosophilidae, commonly called fruit flies, have been exceptionally useful in scientific research. Their short reproductive cycle (a new generation of adults develops in only two weeks) and uncomplicated genetics make them ideal subjects for studies of heredity. This fly lays its eggs in rotting fruit and therefore does not need the stiff, sharp ovipositor that other fruit flies use to drill into fruits and stems.

Great Diving Beetle
The great diving beetle has adapted to an aquatic environment. A voracious predator, the diving beetle feeds on tadpoles, small fish, and other insects and their larvae. Not strictly limited to the aquatic environment, diving beetles are capable of flying from one pool to another.






Hornet
The European hornet is the wasp Vespa crabro, common on the east coast of the United States, where it is also called the giant hornet. This wasp, introduced from Europe in the mid-1800s, constructs its nest of hexagonal cells inside hollow trees. Wasp nests are paper, made of partially digested wood and plant fiber. Hornets may emerge from their nests at night and cluster around sources of light. Like other wasps, hornets have a painful sting.




Katydid
The katydid gets its name from the characteristic sound made by males as they rub their wings together to attract females. It is a tree-dwelling insect in the grasshopper family. The katydid resembles a folded green leaf in both color and appearance and uses this mimicry to escape detection from predators. Pictured here is an insect of the scudderia species.

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