Cephalopods belong to the phylum Mollusca which also includes the bivalves and gastropods. The following subclasses are:

• Ammonoidea


• Nautiloidea


• Coleoidea which includes:

-Order Sepiida: squids and cuttlefish
-Order Teuthida: squids
-Order Octopoda
• Orthoceratoidea: long, straight shells, whose exterior was covered with fine striations


• Actinoceratoidea: had large straight shells, and a complicated system of vessels used for the exchange of gases between chambers


• Endoceratoidea: includes the largest of all Paleozoic shells, had a broad siphuncle
• Bactritoidea: resembled Ammonoids in having a bulbous protoconch

Cephalopods are probably the most elaborately evolved mollusk, and there are 10,000 described species and 400 extant species known. This class holds the record for the largest invertebrate, the giant squid,Architeuthis dux, and has the most highly developed nervous system to be found in invertebrates.

Figure 1: Size comparison between a man(6 ft.) and the Giant squid(59 ft.)

Cephalopods are marine carnivores and are widely distributed in the present oceans and seas. An interesting characteristic about cephalopods is their capability to be buoyant. They can leave the substrate as a bottom dweller capable of locomotion and be nektic as well.

Figure 2: A Nautilus swims to the surface from the sea bottom.

© Sarah Vandervlugt. 1998