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The body cavity of these usually small to microscopic, worm-shaped animals is a pseudocoelom, and they lack a circulatory system. Most pseudocoelomates are often grouped into a single phylum, Aschelminthes, with several classes. Alternatively, as here, each class is a phylum, and Aschelminthes is a superphylum. This leaves one wormlike group (Acanthocephala) and one group of “moss animals” (Endoprocta) outside the aschelminths proper.

Phylum Nematoda (roundworms)

These elongate worms have a firm covering called a cuticle and a body supported by fluid under pressure. They generally feed by sucking fluid or taking in small particles or soft materials. Abundant but inconspicuous, they live in soil and marine and freshwater sediments. Some, as parasites, cause serious diseases. 10,000 species.

Phylum Gastrotricha

These are tiny (less than 1 mm/0.04 in long), short-bodied, marine and freshwater worms. 170 species.

Phylum Nematomorpha

Worms of this minor group resemble roundworms. They are parasitic in arthropods when young and briefly free-living and aquatic as adults. 230 species.

Phylum Acanthocephala (spiny-headed worms)

Parasitic as adults in the guts of vertebrates, and as juveniles in the tissues of various animals, these worms lack a gut and have a spiny attachment organ on the head. They superficially resemble tapeworms. 500 species.

Phylum Kinorhyncha

These tiny (less than 1 mm/0.04 in), short-bodied marine worms have spiny bodies and a spiny proboscis. 100 species.

Phylum Rotifera (wheel animalcules)

Rotifers are so named because of the wheel-shaped organ on the head, used in feeding and swimming. Often microscopic, these animals are abundant in fresh water. 1500 species.

Phylum Priapulida

These free-living marine worms have spiny heads. 8 species.

Phylum Endoprocta (moss animal)

Very small, stalked, marine or freshwater animals, endoprocts feed with tentacles. 75 species.

Phylum Loricifera

This phylum, established in 1983, consists of marine sediment dwellers only 0.5 mm (0.02 in) long. Free-swimming larvae and sedentary adults have mouth cones that can be retracted into a spiny head, itself retractable.

Eucoelomates: the Tentaculata

These coelomate animals have a short body and a crown of tentacles called a lophophore.

Phylum Phoronida

Phoronids have a U-shaped gut because the lower part of the body is greatly elongated. They live in tubes, with the lophophore and anus at the open end. Adults are about 1 to 10 cm (about 0.4 to 4 in) long, and all phoronids are marine. 18 species.

Phylum Ectoprocta, or Bryozoa (moss animal)

The ectoprocts, polyzoans, or true bryozoans are small, colonial animals that resemble simplified phoronids. Almost all inhabit a hardened covering and are primarily marine. 3500 species.

Phylum Brachiopoda (lampshells)

These animals superficially resemble clams but have upper and lower shells rather than left and right ones. All are marine. 230 species.

Eucoelomates: the Trochozoa

The following phyla frequently possess a juvenile stage called a trochophore larva. They all may be derived from an annelidlike ancestor, judging from their widespread metamerism (segmentation) and the frequent occurrence of paired nerve cords ventral to the gut.

Phylum Annelida

Annelids have a well-developed coelom, a soft body, and (as a rule) well-developed metamerism. Bristlelike structures (setae) used in crawling are often present. The group includes earthworms (class Oligochaeta), leeches (class Hirudinea), and the less familiar bristle worms (class Polychaeta). Some animals of uncertain position, here treated as classes of annelids, are sometimes made separate phyla: Echiura and Pogonophora (beardworms). Two minor groups within the polychaetes, Archiannelida and Myzostomida, are often considered independent classes. 8700 species.

Phylum Sipuncula (peanut worms)

These moderate-sized (about 3 cm/1.2 in) marine worms have a saclike body and a long proboscis. They may be simplified annelids. 250 species.

Phylum Mollusca

The second largest animal phylum, including snails and clams, mollusks typically have a hard shell and a soft body. Few traces of metamerism are observed, and the coelom is small; the main body cavity is part of the circulatory system. Some forms, such as octopuses and squid, attain considerable size. The seven mollusk classes are Aplacophora, Polyplacophora (chitons), Monoplacophora, Gastropoda (snails and slugs), Bivalvia (clams and allies), Cephalopoda (octopuses, squid, and allies), and Scaphopoda (tusk shells). 50,000 species.

Phylum Arthropoda

The body of an arthropod is covered with a hard, jointed skeleton. This is the largest animal phylum, partly because so many kinds of insects exist. Arthropods are abundant and successful in almost all habitats. The body remains segmented, and the coelom has been reduced. Three small, transitional groups are sometimes included in the Arthropoda and sometimes made independent phyla: Onychophora, Tardigrada, and Pentastomida. The classes of higher arthropods can be grouped into assemblages. Those with jaws (mandibulates) include the Crustacea (lobsters, crabs, and allies), the allied Myriapoda (centipedes and millipedes), and Insecta (insects). Arthropods with mouthparts called chelicerae include the Pycnogonida (sea spiders), Merostomata (horseshoe crabs), and Arachnida (spiders and their allies). 838,000 species.

Eucoelomates: the Deuterostomia

This large group includes all the remaining animals. The group's name indicates that the mouth is produced secondarily, as a new formation, in embryological development.

Phylum Chaetognatha (arrowworms)

These animals are small, active, exclusively marine swimmers of uncertain relationship. 50 species.

Phylum Echinodermata

Echinoderms derive their name from their spiny skins. The group includes starfish and sea cucumbers. They are slow moving and rely mainly on small processes called tube feet for locomotion. All are marine. The five extant classes are as follows: Crinoidea (feather stars), Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins), and Holothuroidae (sea cucumbers). 6000 species.

Phylum Hemichordata (acorn worms)

These simple, wormlike marine animals have a few characteristics linking them to chordates. Among these are traces of what may be a notochord (see discussion under Phylum Chordata) and a dorsal nerve cord, as well as a system of gill slits. The larval body of the worms, however, indicates a link to the echinoderms. 230 species.

Phylum Chordata

Chordates include vertebrates (animals with backbones) and some related invertebrates. At some time in their lives, all possess a stiff rod, called a notochord, that lies above the gut. In vertebrates, a series of bones (vertebrae) replaces the notochord.

Two subphyla of invertebrate chordates exist. The Tunicata (tunicates, or sea squirts) become highly modified as adults and attach to a substrate, feeding with gill slits. Only the free-swimming larvae retain a notochord. All are marine, and about 1300 species exist. The Cephalochordata (lancelets) resemble very simple fish. They live in the sea and extract food from the water with their gill slits. About 25 species exist.

The remaining Chordata belong to the subclass Vertebrata and have a backbone. Half of the 42,000 species are fish. Thus, although the group is very successful, it is outnumbered by arthropods and mollusks. Seven classes are generally recognized. The old class Pisces (fish) has long since been broken down into three classes: Agnatha (jawless fish such as lampreys), Chondrichthyes (cartilaginous fish such as sharks and rays), and Osteichthyes (bony fish). The class Amphibia contains such semiaquatic forms as salamanders, toads, and frogs. The class Reptilia is better adapted to life on land; it includes turtles, lizards, and crocodilians. The class Aves (birds) is noteworthy not only for flight but also for warm blood and an insulating layer of feathers. The class Mammalia is characterized by the possession of hair and of mammary glands that secrete milk. This group also is warm-blooded.

Animals in the Ecosystem

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