Shellfish come triploblastic protostomes. A chief person cavity occurs as blood-filled hemocoel, with an actual coelom present but reduced to trace in a hearts, gonads, and metanephridia (kidney-like organs). a system is divided into the head, typically sustaining eyes or even tentacles, the muscular foot & a intuitive mass housing the organs. Covering a system occurs as heavy sheet known as a mantle, which in virtually all forms secretes the chalky shell.

Shellfish have the mantle, which occurs as stack of a outer skin lining the casing, & a muscular foot that is utilized for motion. Numerous mollusc keep around their mantle develop the calcium carbonate external shell & their gill extracts oxygen from either a water supply & disposes waste. Everthing coinage of a phylum Mollusca have a complete digestive tract that starts from either the mouth to the arse. Numerous have a radula, mostly composed of chitin, in a mouth, which allows then to scrape food from either the surface by sliding back & forth. Mollusc as well have a coelom, made from either cell people, in which a lot organs come suspended. Unlike a closely related annelids, mollusks lack system segmentation.

Development lives across of these or even deuce trochophore stages, one of which (the veliger) is unique to the group. These indicate a close relationship between a shellfish & various more protostomes, notably the Annelids. Shellfish fossils come a bit of of the better known & are uncovered from either a Cambrian onwards. There are eight dwelling classes and one class, known simply from either fossils:

Class Caudofoveata (deep-sea wormlike animals; Seventy known metal money); today typically recognized as a subclass of Aplacophora. Class Aplacophora (solenogasters, deep-sea groveling animals; 250 coinage) Class Polyplacophora (chitons; 60Mintage, bumpy marine shorelines) Class Monoplacophora (deep-sea limpet-like animals; Xi dwelling coinage) Class Bivalvia (also Pelecypoda) (clams, oysters, scallops, mussels; 8,00Metal money) Class Scaphopoda (tusk shells; 350 mintage, tons marine) Class Gastropoda (nudibranchs, snails and slugs, limpets, sea hares; sea angel, sea butterfly, Sea Lemon; estimated 40,000 - 150,000 coinage) Class Cephalopoda (squids, octopuses, nautilus, cuttlefish; 786 species, 100% marine) Class † Rostroconchia (fossils; probably more than 1,000 coinage; likely ascendant of lamellibranch)

Brusca & Brusca (1990) suggest that the bivalves and scaphopods are sister groups, as are the gastropods and cephalopods, so indicated in the relationship diagram above.

In that Phylum's level of organization, organ systems from either everthing ternion primary germ shells may be discovered: