Photos of Mammals

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Aardwolf
The aardwolf, a member of the hyena family, is found throughout southern and eastern Africa. Aardwolves travel at night hunting for food, which consists of carrion (dead and decaying animal flesh) and insects, usually termites. They have weak jaws and small teeth, leaving them without defenses against enemies such as dogs.

Abyssinian
Abyssinians are particularly affectionate and have fine, soft fur resembling that of a wild rabbit. The origin of the cat is unknown, although its appearance suggests a relationship to the cats of ancient Egypt. It is one of the oldest breeds of domesticated cat.




Afghan Hound
The Afghan hound, similar to the greyhound, originated in Afghanistan and was known in ancient Egypt. A capable hunting dog, the Afghan hound is a swift runner and a powerful jumper.








African Elephants
African elephants live in grassy regions south of the Sahara Desert. Bull (male) elephants stand about 3.4 m (11 ft) tall and weigh about 5.4 metric tons, while cow (female) elephants stand 2.8 m (9 ft) tall and weigh about 3.6 metric tons. They have no sweat glands, so they like to cool off by rolling in ponds and streams. The mud that dries on their skin protects it from the sun.



Agouti
A large rodent common to the forests of Central America and central-northern South America, the swift, ground-dwelling agouti feeds on leaves, roots, and fallen fruit. The agouti is hunted by farmers because it destroys crops in agricultural areas.






Akhal-Teké
A prized desert horse, the Akhal-Teké has been bred and raised for at least 3000 years, and may have been an ancestor of the Arabian. Remarkable endurance and resistance to heat give the powerful yet graceful Akhal-Teké an edge in competitive long-distance riding and show jumping.



Alaskan Malamute
This Alaskan Malamute has dense, layered fur as a shield from severe weather. The nomadic Mahlemuts, an Inuit people of northwestern Alaska, first bred the Alaskan Malamute as a sled dog to haul their belongings. Ranging from light gray to black in color, Alaskan Malamutes are also popular as pets and show dogs.







Alpine Ibex
Famous for its surefootedness in steep, rocky terrain, the Alpine ibex is one of four species of mountain goat in the genus Capra. The Alpine ibex was on the brink of extinction from overhunting and habitat destruction in the early 20th century, but is now protected and rebuilding herd populations in Italy’s Gran Paradiso National Park and other European wildlife preserves.





American Black Bear
The American black bear lives in forests throughout North America, from Alaska and Canada to mountains in northern Mexico. Unlike most other bear species, the American black bear has adapted to human encroachment on its habitat, and it is able to survive in populated suburban areas. Despite the bear’s common name, the color of the American black bear ranges from black to brown, cinnamon, beige, and even pure white.






Antarctic Fur Seal
The Antarctic fur seal is one of 14 species of eared seals. Fur seals are polygamous and assemble in huge breeding colonies beginning in October. Breeding females feed exclusively on small, shrimplike crustaceans called krill, whereas males feed on krill, penguins, and fish. Although most eared seal populations have been decimated by overhunting, the Antarctic fur seal has rebounded from near extinction in the late 1800s to a current population of between 700,000 and 1 million animals.

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Coral

Coral, common name for members of a large class of marine invertebrates characterized by a protective calcium carbonate or horny skeleton. This protective skeleton is also called coral. Corals are divided into two subclasses, based on differences in their radial symmetry (symmetry around a central axis). One subclass consists of colonial, eight-tentacled animals, each with an internal skeleton. Among them are whip corals, gorgonians, and the red coral used in making jewelry. Members of the other subclass commonly have six tentacles, or multiples of six, but other patterns are also observed. They include the stony, or true, corals. Another class of the same phylum also contains forms known as coral that are not considered here.

STRUCTURE
True corals secrete calcium carbonate from the bottom half of the stalk of the individual animal, or polyp, forming skeletal cups to which the polyps are anchored and into which they withdraw for protection. In the flattened oral disc at the top of the stalk is an opening, edged with feathery tentacles and cilia, that is both mouth and anus. At night the tentacles extend from the cup, seize animal plankton that wash against them, and carry the food to the mouth. Stinging cells, or nematocysts, on the tentacles can also paralyze prey.

COLONIES
Some scleractinians exist as solitary polyps, but the majority are colonial. Colonial polyps average from 1 to 3 mm (0.04 to 0.12 in) in diameter. They are connected laterally by tubes that are an extension of the polyps' gastrovascular cavities, and the colony grows by asexual budding from the base or the oral disc of the polyps. Living polyps build on the deposits of their predecessors; the wide range of branched or massive forms that result depends on the species involved.

Colonial corals can grow in deep water, but reef-building corals are found only in warm, shallow seas. They live no deeper than light can penetrate because the symbiotic algae called zooxanthellae that live in their tissues require light for photosynthesis, and the corals cannot exist without the algae. Carbon is passed by the algae to the coral, increasing its energy, and the food caught by the coral may supply nitrogen and phosphorus for both organisms. The dependence of the corals on the algae probably varies according to species and locality. Scleractinian corals are the main contributors to a reef, but other organisms also add to it, such as hydrozoan corals, calcareous algae, mollusks, and sponges. See Coral Reef.

Scientific classification: Corals belong to the class Anthozoa in the phylum Cnidaria. Corals with eight tentacles make up the subclass Ostocorallia or Alcyonaria. Corals that commonly have six tentacles or multiples of six belong to the subclass Zoantharia (or Hexacorallia). Stony, or true, corals belong to the order Scleractinia (or Madreporaria).

Silkworm

Silkworm, common name for the silk-producing larvae of any of several species of moths. Silkworms possess a pair of specially modified salivary glands called silk glands, or sericteries, which are used in the production of cocoons. The silk glands secrete a clear, viscous fluid that is forced through openings, called spinnerets, on the mouthparts of the larva; the fluid hardens as it comes into contact with air. The diameter of the spinneret determines the thickness of the silk thread produced.

The best-known silkworm is the larvae of the common, domesticated silkworm moth. This moth, native to China, was introduced into Europe and western Asia in the 6th century ad and into North America in the 18th century. The moth has been cultivated for many centuries and is no longer known in the wild state. Breeders have produced many varieties of the moth, the most important of which produce three broods of young annually.

A typical adult silkworm moth is yellow or yellowish-white, with a thick, hairy body, and has a wingspread of about 3.8 cm (about 1.5 in). The adult has rudimentary mouthparts and does not eat during the short period of its mature existence; the female dies almost immediately after depositing the eggs, and the male lives only a short time thereafter. The female deposits 300 to 400 bluish eggs at a time; the eggs are fastened to a flat surface by a gummy substance secreted by the female. The larvae, which hatch in about ten days, are about 0.6 cm (about 0.25 in) long. The larvae feed on leaves of white mulberry, Osage orange, or lettuce. Silkworm caterpillars (see Caterpillar) that are fed mulberry leaves produce the finest quality silk. Mature larvae are about 7.5 cm (about 3 in) long and yellowish-gray or dark gray in color.

About six weeks after hatching, the common silkworm stops eating and spins its cocoon. The length of the individual fiber composing the cocoon varies from 300 to 900 m (1000 to 3000 ft). The silkworm pupates for about two weeks; if allowed to complete its pupation period, it emerges as an adult moth. Tearing during emergence damages the silken cocoon beyond commercial use. Therefore, in the commercial production of silk, only enough adult moths are allowed to emerge to ensure continuation of the species. Most of the silkworms are killed by heat, either by immersion in boiling water or by drying in ovens.

Other moths known as silkworm moths include the giant silkworm moths. The larvae of these large moths also spin silken cocoons, but they are less widely used for commercial silk production.

Scientific classification: Silkworms belong to the order Lepidoptera. The domesticated silkworm moth makes up the family Bombycidae and is classified as Bombyx mori. The giant silkworm moths make up the family Saturniidae.

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.