Cephalopoda

This is a completely marine class of mollusc which includes includes octopus, cuttlefish and squids. The group represents some of the most sophisticated invertebrates in existance. The molluscan foot has developed into a set of arms armed with suckers which surround the mouth. These are used to capture prey which is then passed to the mouth where it is initially bitten by a set of beak like jaws before being further broken up by the radula (a ribbon of teeth). They are mainly pelagic, swimming by jet propulsion, the water being expelled from the mantle cavity. 

With the exception of the more tropical Nautiloids, the shell has been reduced or lost altogether (as is the case of the octopuses). They generally have a single pair of ctenidia or gills which extract oxygen from the water. These lack cilia; the water is moved across them through muscular pumping. They have a closed blood system with the blood being pumped around the body by one or a number of hearts.

The nervous system is very advanced for an invertebrate. The complex eyes have the same general form as found in vertebrates having a cornea, iris diaphagm, lens and retina. In addition the nervous system also controls the muscles around surface chromatophores which allow the group to change colour very rapidly.

The sexes are separate and fertilisation is internal. Unusually for molluscs there is no separate larval stage; development is direct.  Interesting the species in this group are generally short lived, breeding once and then dying.