Question

which key characteristic of the mollusca forms the octopus head? and which forms the arms?

Answer 1

Octopus is in cephalopods subgroup of mollusca.

In mollusca, body is divisible into three regions – head, a visceral mass, and ventral foot.The bivalve molluscs are used as bioindicators of the freshwater and marine environments.

These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles (muscular hydrostats) modified from the primitive molluscan foot. Fishermen sometimes call cephalopods "inkfish," referring to their common ability to squirt ink.

Octopuses, squid, and cuttlefish are all cephalopods, which are a type of mollusk. Other types of mollusk are gastropods (snails and slugs) and bivalves (clams and oysters). They all share a common ancestor.