The mollusks are a large and diverse phylum of Animalia, including a variety of familiar creatures, well-known for their decorative shells or as sea food. These range from tiny snails and clams to the giant squid, the largest and one of the most intelligent invertebrates.
Mollusks are triploblastic protostomes. The principle body cavity is a blood-filled hemocoel, with an actual coelom present but reduced to vestiges around the hearts, gonads, and kidneys. The body is divided into a head, often with eyes or tentacles, a muscular foot and a visceral mass housing the organs. Covering the body is a thick sheet called the mantle, which in most forms secretes a calcareous shell.
Development passes through one or two trocophore stages, one of which (the veliger) is unique to the group. These suggest a close relationship between the mollusks and various other protostomes, notably the Annelida. Mollusk fossils are some of the best known and are found from the Cambrian onwards. They are divided into eight classes, unless I am missing some extinct groups:
- Class Caudofoveata
- Class Aplacophora (solenogasters)
- Class Polyplacophora (chitons)
- Class Monoplacophora
- Class Bivalvia (clams, oysters, etc)
- Class Scaphopoda (tusk shells)
- Class Gastropoda (snails and slugs)
- Class Cephalopoda (squids, octopi, etc)
Brusca & Brusca suggest that the bivalves and scaphopods are sister groups, and otherwise the above sequence represents an evolutionary progression. I haven't seen anything more recent.