|The name cephalopod means 'head-footed' and derives from the
Greek Kephalè (head) and Podos (foot). It is an apt name,
as the head is large and surrounded by eight or more tentacles;
these tentacles have evolved from the anterior part of the molluscan
foot. The diagram below shows the body
plan of a basal mollusc and that of two modern cephalopods.
The evolution of cephalopods was based on a variety of innovative designs and major structural changes. By developing the chambered shell, cephalopods were able to free themselves from the seafloor and colonise the vast oceans above. In today's cephalopods the body may be protected within a shell (Nautilus), strengthened by an internal skeletal structure (cuttlefish and squid) or be absent entirely (octopi).
Image from Clarkson.
The sexes are separate and fertilization is internal. The male inserts a bundle of sperm (spermatophore) into the female and in most species the male has one specially modified tentacle, the hectocotylous arm, for this purpose. The female then lays the eggs in clusters or strings within protected crevices, on seaweeds or under boulders, after a few weeks the eggs hatch and the juveniles emerge as miniature versions of their parents.