Zoology

The first molluscs probably arose during Precambrian times because fossils attributed to Mollusca have been found in geological strata as old as the early Cambrian period. On the basis of such shared features as spiral cleavage, mesoderm from the 4d blastomere, and trochophore larva, many zoologists argue that Mollusca are …

Class Oligochaeta More than 3000 species of oligochaetes are found in a great variety of sizes and habitats. They include the familiar earthworms and many species that live in freshwater. Most are terrestrial or freshwater forms, but some are parasitic, and a few live in marine or brackish water. With …

Bivalvia are also known as Pelecypoda (pel-e-sip o-da), or “hatchet-footed” animals, as their name implies (Gr. pelekys, hatchet, pous, podos, foot). They are bivalved molluscs that include mussels, clams, scallops, oysters, and shipworms and they range in size from tiny seed shells 1 to 2 mm in length to giant …

Cephalopoda (Gr. kephal¯e, head, + pous, podos, foot) include squids, octopuses, nautiluses, devilfish, and cuttlefish. All are marine, and all are active predators. Their modified foot is concentrated in the head region. It takes the form of a funnel for expelling water from the mantle cavity, and the anterior margin …

Clam Worms: Nereis Clam worms, or sand worms as they are sometimes called, are errant polychaetes that live in mucous-lined burrows in or near low tide. Sometimes they are found in temporary hiding places, such as under stones, where they stay with their bodies covered and their heads protruding. They …

The largest class of annelids is the Polychaeta (Gr. polys, many, chaite¯, long hair) with more than 10,000 species, most of them marine. Although most polychaetes are 5 to 10 cm long, some are less than 1 mm, and others may be as long as 3 m. They may be …

Respiration in most gastropods is performed by a ctenidium (two ctenidia is the primitive condition, found in some prosobranchs) located in the mantle cavity, though some aquatic forms lack gills and instead depend on the mantle and skin. After some prosobranchs lost one gill, most of them lost half of …

Traditional classification of class Gastropoda recognizes three sub classes: Prosobranchia, the largest subclass, almost all of which are marine; Opisthobranchia, an assemblage including sea slugs, sea hares, nudibranchs, and canoe shells, all marine; and Pulmonata, containing most freshwater and terrestrial species. Currently, gastropod taxonomy is in flux. Evidence suggests that …

Phylum Annelida (an-neli-da) (L. annelus, little ring, ida, pl.suffi x) consists of the segmented worms. It is a diverse phylum, numbering approximately 15,000 species, the most familiar of which are earthworms and freshwater worms (class Oligochaeta) and leeches (class Hirudinida). However, approximately two thirds of the phylum comprises marine worms …

Although a fluid-filled coelom provided an efficient hydrostatic skeleton for burrowing, precise control of body movements was probably difficult for the earliest coelomates. The force of muscle contraction in one area was carried throughout the body by the fluid in the undivided coelom. In contrast, there were distinct coelomic compartments …