Class CEPHALOPODA

The cephalopods are a class of mobile mollusks, most of which are nektic or nekto-benthic. Cephalopods have a bilaterally symmetrical body, a prominent head, and a modified foot in the form of tentacles. Although during the Paleozoic and Mesozoic, cephalopods achieved great diversity and abundance in marine habitats, only two genera possessing skeletons are known today. See the example of a Nautilus. Superficially the shell or conch of cephalopods resemble gastropods; however, most cephalopods coil in a plane, whereas gastropods are helicoiled. Furthermore, in cephalopods with an external conch, the coiled shell is chambered.

Cephalopod Morphology

Some morphologic terms you should become familiar with are given below.

Phragmocone or conch: the external chambered shell.

Septum (plural septa): an internal partition which separates the chambers.

Living chamber: the space between the aperture and the last septum.

Siphuncle: the tube connecting the living chamber with all previous chambers. The siphuncle is the plane of bilateral symmetry. See the example of the siphuncle on the Recent Nautilus.

Suture: the outer edge of the septum (or juncture of septum with shell wall) which usually is expressed in outer wall of shell. The suture can be relatively straight as in Nautilus. or fluted with saddles and lobes, saddles are convex toward the direction of growth whereas lobes are concave. (See figure under subclass Ammonoidea).

Ribs: thickenings of external shell that may not be coincident with sutures.

Keel: thickening along the outer (venter) margin.

Shape of External Shells

Several different shapes are common among fossil and extinct cephalopods. These include orthoconic or straight, brevicone, evolute planispiral, involute planispiral, or heteromorphic.

Morphology of Internal Shells

By definition, internal shells were surrounded by flesh during their development. Thus, they are commonly solid plates or nearly solid. Some terms that apply to internal shells include the phragmocone - the conical cavity in the anterior end of a belemnoid; and the rostrum - the part of a belemnoid enclosing the phragmocone and extending posterior of it. The rostrum is solid calcite composed of radially arranged fibers exhibiting concentric growth bands.

Nonskeletal hardparts

A variety of non skeletal hard parts may be associated with cephalopods including Aptychi, which may serve as an operculum. Cephalopods may also have a beak and radula to aid in obtaining food.

Subclass COLEOIDEA

The Coleoids are perhaps the most familiar cephalopod mollusks including as they do the octopods and squids. Coleoids are characterized either by an internal skeleton or by lacking a skeleton altogether. The internal shell of coleoids is almost exclusively straight (=orthoconic), although a few groups have a coiled shell. Others have a more complicated pattern such as in the cuttlebone. The internal skeleton may consist of two parts, the outer rostrum and inner phragmocone as typified in the belemenites an order of squid-like animals which produced cigar shaped rostrum which has a conical depression at one end and a central cone-like phragmocone which is rarely found.

Subclass NAUTILOIDEA

As with the ammonoids (see below), the nautiliods are an important group of cephalopods with an external shell. However, unlike the ammonoids, the nautiloids have living representatives in the genus Nautilus. Nautiloid shells are external and are characterized by either straight or slightly wavy sutures. Nautiloid shells are either orthoconic, or they are coiled, such as the Recent Nautilus; see also the fossil examples. The siphuncle may be small or large, but is typically centrally located.

Subclass AMMONOIDEA

This is a very important extinct group of cephalopods which includes all forms with an external shell with fluted septa. Most are planispiral, but some may be heteromorphic (= not planispiral which can include orthoconic or a variety of shapes). The siphuncles are generally small and ventral in position. Division within the ammonoids is based upon the grades of suture fluting. There are three grades you will need to know which are illustrated and described below: