Rotifers are dioecious, and males are usually smaller than females. However, despite having separate sexes, males are entirely unknown in the class Bdelloidea, and in the Monogononta they seem to occur only for a few weeks of the year. The female reproductive system in the Bdelloidea and Monogononta consists of …

Chitons (Gr. coat of mail, tunic)  repreent a somewhat more diverse molluscan group with about 1000 currently described species. They are rather flattened dorsoventrally and have a convex dorsal surface that bears seven or eight articulating limy plates, or valves, hence their name Polyplaophora (“many plate bearers”). The plates overlap …

For more than 50 years five classes of living molluscs were recognized: Amphineura, Gastropoda, Scaphopoda, Bivalvia (also called Pelecypoda), and Cephalopoda. Discovery of Neopilina in the 1950s added another class (Monoplacophora), and Hyman contended that solenogasters and chitons were separate classes (Aplacophora and Polyplacophora), lapsing the name Amphineura. Subsequently, Aplacophora …

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 …