Phylum Mollusca

Phylum Mollusca

Molluscs are soft-bodied ("mollis" = soft) animals, typically unsegmented, usually protected by a calcareous shell secreted by a specialised sheet of tissue called the mantle, and showing enormous diversity of form across its several classes.

• Class Aplacophora / Polyplacophora (Chiton) — Chiton (a member of class Polyplacophora, sometimes grouped near the shell-less Aplacophora in broad surveys as one of the more primitive molluscan lineages) is identified by its dorsal shell made up of eight separate, overlapping calcareous plates arranged in a row along the back, a broad, flattened, muscular foot used to cling tightly to rocky surfaces, and a generally elongated, oval body form — the segmented-looking row of eight shell plates is the single most distinctive identifying feature of Chiton.
• Class Gastropoda — the snails and slugs, typically bearing a single, usually coiled shell (though reduced or absent in slugs) and a rasping feeding structure called the radula, a chitinous, tongue-like ribbon studded with many rows of small teeth. The radula functions by being drawn repeatedly back and forth over a food surface (algae, plant tissue, or other organic material), scraping and shredding it into small particles that can then be ingested — a feeding mechanism essentially unique to Mollusca among all the phyla surveyed in this chapter.
• Class Bivalvia — molluscs enclosed within a shell of two hinged valves (e.g. clams, mussels, oysters), lacking a radula, and instead feeding by filtering suspended food particles from water drawn across their gills. The mollusc gill is also known by the alternate name ctenidium (plural ctenidia) — a comb-like respiratory structure that, in bivalves, additionally plays a major role in filter-feeding, not merely gas exchange. …