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Mollusks Phylum Mollusca Mollusks • Mollusk: soft-bodied organisms that have either an internal or external shell – Belong to the Phylum Mollusca which means “soft.” – Found living in both aquatic and terrestrial environments – Are invertebrates Development • Many mollusks share similar developmental stages. •Many aquatic mollusks have a free-swimming larval stage called a trochophore. •The trochophore larva is also characteristic of annelids, indicating that these two groups may be closely related. Body Plan • The body plan of most mollusks has four parts: foot, mantle, shell, and visceral mass • The muscular foot takes many forms for crawling, burrowing or capturing prey Feeding • Mollusks can be herbivores, carnivores, filter feeders, detritivores, or parasites. • Have a structure called a radula that is used to obtain food – Can be used to scrape food off of the groundsnails and slugs – Can be used to penetrate and rip flesh like octopus and squid Respiration • Aquatic mollusks breathe using gills inside their mantle cavity. • However in some freshwater snails their gills are reduced, instead leaving the snail the thin wall of the mantle cavity to assimilate oxygen through. This organ is called the snails' lung Gills Circulation and Excretion • In an open circulatory system, blood is pumped through vessels by a simple heart. Heart • Nephridia remove ammonia from the blood and release it outside the body. Nephridia Mollusks • Groups of Mollusks – A) Gastropods • Shell-less or single shelled mollusks that move by a muscular foot located on the ventral (bottom) side – Ex: snails and slugs – B) Bivalves • Have two shells that are held together by powerful muscles – Ex: clams, scallops, and oysters – C) Cephalopods • Mollusk in which the head is attached to a single foot that is usually divided into tentacles – Ex: octopus and squid