Sipuncula

.
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

.
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

.
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

.
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

.
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.