Phylum
Mollusca
A Pacific giant octopus, Octopus
pacificus, swimming by means of water jets seen at the base of the mantle. Photo by Dr. R. G. Sprackland.
Mollusks
represent the second largest phylum of animals known, second only to the
Arthropoda. There is a huge range of variety among mollusks, from familiar
clams, scallops and mussels to snails, slugs, sea slugs, octopuses and
squids.
The molluscan body plan includes a huge foot
(modified into arms and tentacles in cephalopods), a rasping tongue
called a radula, and the mantle, which secretes the shell material in most
species. Excepting most cephalopods, the mollusks have open circulatory
systems and, usually, three-chambered hearts. Mollusks are also protostomes¹
and true coelomates.
There is also tremendous range of size among
mollusks, from centimeter-long bivalves (calms and scallops) and gastropods
(snails and slugs) to giant clams and giant squids.
Perhaps most intriguing is that mollusks
include virtually brainless animals as well as some of the most intelligent
creatures on earth. The learning and social behaviors of some octopuses and
squids exceeds that of many vertebrates. There are eight classes recognized.
