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.