Mollusca (mol-lus’ ka) (L. molluscus, soft) is one of the largest animal phyla after Arthropoda. There are over 90,000 living species and some 70,000 fossil species. Molluscs are coelomate lophotrochozoan protostomes, and as such they develop via spiral mosaic cleavage and make a coelom by schizocoely. The ancestral larval stage …

Brachiopoda (brak-i-op’ – o-da) (Gr. brachio¯n, arm, pous, podos, foot), or lamp shells, are an ancient group. Although about 325 species are now living, some 12,000 fossil species, which once flourished in Paleozoic and Mesozoic seas, have been described. Modern forms have changed little from early ones. Genus Lingula (L. …

The enormous variety, great beauty, and easy availability of shells of molluscs have made shell collecting a popular pastime. However, many amateur shell collectors, even though able to nameh undreds of the shells that grace our beaches, know very little about the living animals that created those shells and once …

Ectoprocta (ek-to-prok’ – ta) (Gr. ektos, outside, + proktos, anus) contains aquatic animals that often encrust hard surfaces. Most species are sessile, but some slide slowly, and others crawl actively, across the surfaces they inhabit. With very few exceptions, they are colony builders. Each member of a colony is small, …

Members of phylum Acanthocephala (a-kan’ -tho-sef ‘-a-la) (Gr.akantha, spine or thorn, + kephale¯, head) are commonly called “spiny-headed worms.” The phylum derives its name from one of its most distinctive features, a cylindrical, invaginable proboscis bearing rows of recurved spines, by which it attaches itself to the intestine of its …

The final three phyla in this chapter are the most controversial taxa placed within Protostomia. Evidence that Ectoprocta, Brachiopoda, and Phoronida belong within the lophotrochozoan subgroup of protostomes comes from sequence analysis of the genes encoding small-subunit ribosomal RNA. Some developmental data are consistent with the molecular data: in phoronids, …

Entoprocta (en’ – to-prok ‘-ta) (Gr. entos, within, + proktos, anus) is a small phylum of about 150 species of tiny, sessile animals that superficially resemble hydroid cnidarians but have ciliated tentacles that tend to roll inward. Most entoprocts are microscopic, and none is more than 5 mm long. They …

Gastrotricha ( gas-tro-tri’ -ka) (N. L. fr. Gr. gaster, gastros, stomach or belly, + thrix, trichos, hair) includes small, ventrally flattened animals usually less than 1 mm in length. The largest species of gastrotrichs can reach lengths of about 3 mm. Superficially, gastrotrichs may appear somewhat like rotifers but lacking …

The first and only micrognathozoan , Micrognathozoa specimen, was collected from Greenland in 1994 but not formally described until 2000. Micrognathozoans are tiny animals that are interstitial (living between sand grains) and about 142 µ m long. The body consists of a two-part head, a thorax, and an abdomen with …

Rotifera (ro-tif -e-ra) (L. rota, wheel, fera, those that bear) derivet heir name from the characteristic ciliated crown, or corona, that, when beating, often gives the impression of rotating wheels . Rotifers range from 40 µ m to 3 mm in length, but most are between 100 and 500 µ …