The phylum is also called Cnidaria, especially when extended to include the Ctenophora (see below). The stages of a coelenterate’s life history include an attached polyp (as in sea anemones), a free-swimming medusa (as in jellyfish), or both. Prey is taken with stinging capsules, generally located on the tentacles. The animals are marine, except for a few freshwater forms such as Hydra. 3 classes and 5300 species.

Phylum Ctenophora (comb jellies)

These marine animals resemble true jellyfish but have no polyp stage, and they swim using bands of hairlike cells resembling combs. Prey is usually taken with sticky tentacles. 80 species.

The Acoelomate Bilateria

The two major phyla consist of small to moderate-sized worms with bilateral symmetry and no coelom.

Phylum Platyhelminthes (flatworms)

These worms have no anus or circulatory system and are structurally simple, although the generally hermaphroditic reproductive system is often quite intricate. Their flat shape is necessary because the tissues must be near the surface to facilitate gas and nutrient exchange with the environment. The free-living class Turbellaria is abundant in the sea and fresh water and rare on land. Two classes are parasitic: Trematoda (flukes) and Cestoda (tapeworms). An obscure marine group, Gnathostomulida, is sometimes put here and sometimes made a separate phylum. 13,000 species.

Phylum Nemertina, or Rhynchocoela (ribbon worms)

Elongate worms with a circulatory system and an anus, ribbon worms are common in the sea and rare on land. Prey is taken with an extensible proboscis. 800 species.

The Pseudocoelomates

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.