The name cephalopodan has been derived from Greek words cephalic = head + podos = foot

The name cephalopodan has been derived from Greek words cephalic = head + podos = foot. They are more evolved than pelecypods and gastropods.

These exclusively marine invertebrates are represented by the living natilus, octopus and squid.

The living forms have a bilaterally symmetrical body with well developed head encircled by arm like processes near its mouth out of which two are longer than the others and are used for locomotion and catching of food, eyes, mouth with jaws, radula, digestive and circulatory systems, gills for respiration and nerve rings

Based on the presence or absence of a shell and its nature, the cephalopods are divided into subclasses Nautiloidea, Ammonoidea and Dibranchia. The Nautili’s and Ammonoids have external shells while the Dibranchia (or Coleoids) have either an internal shell or no shell at all.

Morphology: The Nautiloidea shell is a conical tube like structure. The shell is closed at the pointed posterior end and opens at the wider anterior end. The ventral part is marked by the presence of a depression known as hyponomic sinus along the margin of the aperture and the surface opposite to it is dorsal part.

The shell may be straight or coiled or may be partially coiled and partially straight or even slightly curved. The coiling is commonly in a horizontal plane but in some forms the coiling is helicoidal. Each coil of 360° is known as a whorl.

In a coiled shell, the whorls may be in touch with each other or the last whorl completely embraces the earlier whorls while in others each whorl may remain separated from the other whorls.

The interior of the shell is divided into a number of chambers by means of transverse partitions known as septa. The septa are generally concave towards the aperture of the shell, however, there are exceptions.

The outer shell starts with the embryonic protoconch and grows with addition of septa as the animal grows in age. After the formation of septa the animal comes to live in the last chamber, which is known as the body chamber! Body chamber is always larger than the earlier chambers and is near the aperture.

This process goes on and shell thus grows in size becomes multi-chambered. All the chambers except the body chamber] filled with air. These chambers are known as air chambers. The air in chambers helps the animal to maintain equilibrium while floating diving in water.

All the chambers are traversed by a slender cord like siphuncle consisting of the sepal neck and connecting ring. The siphuncle starts from the body] chamber passing through the centre or periphery of the septa and extend up to the first formed chamber. At the point where the umbilicus pierces the septa, a short tube like structure is produced on the septa called sepal neck.

The sepal neck in some forms found directed towards the aperture of the shell while in others it may be directed towards the preceding chamber. In Nautili’s the siphuncle passes through the central part of the successive septa, the sepal neck being directed backwards facing the protoconch, whereas, in ammonoids, it is found near the outer margin o the shell with the sepal neck pointing towards the aperture. When isolated each septum appears like a ‘glass funnel’ and the stem of the fund forming the sepal neck.

A number of funnels are arranged according to their size from smaller to larger, the siphuncle passing through their stems appear like a chain of funnels inside a conical outer shell. The outer margin of the septa may be simple or may be systematically folded and crenulated. The line along which the septa are joined with the inner side of the tubular shell is described as suture line.

The nature of suture line differs in different groups of cephalopods. In Nautiloidea the suture line may be simple or slightly undulating, while in Ammonoidea it is generally complex depending on the nature of the margin of the septa. In the complex type of suture line, the convex portion facing towards the aperture is known as saddle while the concave portion in between two saddles is known lobe.

Due to further crenulations the primary saddles and lobes may develop secondary, superior, inferior, auxiliary and inner saddles and lobes starting from the outer margin to the inner margin of the whorl. Basing on their complexity four types of suture line are recognised. These are:

(i) Straight or slightly undulating suture line is known as Nautiloid type.

(ii) Suture line with rounded saddles and angular lobes is known as Goniatite type.

(iii) Suture line with rounded saddles and finely crenulated lobes is known as Ceratite type.

(iv) Suture line with complex saddles and lobes is known as Ammonoid type.