Phylum Mollusca and Phylum Echinodermata

Phylum Mollusca (Latin molluscus, soft) is, after Arthropoda, the second largest phylum of the Animal Kingdom, and is remarkable for the diversity of body forms built on essentially the same soft-bodied, unsegmented, bilaterally symmetrical, triploblastic and coelomate plan. The body is conventionally divided into a distinct head, a ventral muscular foot (used for locomotion, and modified into arms/tentacles in cephalopods such as the octopus and squid), and a dorsal visceral hump enclosing the internal organs. A fold of the body wall, the mantle, envelops the visceral hump and encloses a space, the mantle cavity, that houses feather-like respiratory gills called ctenidia; in most (but not all — bivalves being the exception) molluscs the mantle also secretes an external calcareous shell. Feeding is typically by means of a file-like rasping organ, the radula, used to scrape food off a surface; bivalves such as mussels and oysters, being filter-feeders, have lost the radula entirely. The circulatory system is open (except, notably, in cephalopods, which alone among molluscs have a closed system, an adaptation linked to their unusually active, predatory lifestyle). Familiar examples span several classes: Pila (the apple snail, a gastropod), Pinctada (the pearl oyster, a bivalve), and the cephalopods Sepia (cuttlefish), Loligo (squid) and Octopus. …