CEPHALOPODA

CEPHALOPODA

Cephalopoda - Cephalo "head" and poda "foot", meaning "head-footed". It is most highly organised class of mollusks. This is only class of mulluscks which is entirely Marine.

Characteristic of cephalopods are

• High mobility rate

• Well developed nervous system and brain, and sharp eye sight

• Soft parts posses bilateral symmetry.

• Origin is bit controversial, evidences suggests they were appeared in Ordovician.

• Some forms like Nautilus are living while ammonoids, bellemenites are extinct forms.

• They are characterised by jet propulsion locomotion system.

Basic Morphology

Shell is conical tube like structure with closed end on posterior side. A shell is composed of 3 parts, Protoconch, Phgramocone and Body chamber.

Initial point of shell is called protoconch.

Phragmocone is chambered part. Each chamber is sealed off from previous chambers by a septum. When septum weld to shell, suture develops.

Animal lives in the newest made chamber called body chamber, and thus moves towards the aperture. It is in general largest chamber. 

Hyponomic sinus is a depression which marks ventral surface.

All chambers except body chambers are filled with air to maintain balance of the shell and thus called air chambers. 

Siphuncle tube connects all chambers which pass through centre of septa (as in Nautilus) or pass through a edges of septa (as in Ammonoids).

Coiling can be in horizontal plane called Discoidal as in Nautilus or can be in vertical plane called Helicoidal as in Turrilites.

Nautiloidea

Nautilus is a living and common member of this subgroup. It prey with its powerful jaws. It's shell is aragonatic. 

Living Nautilus has its head, tentacles, foot and hyponome concentrated near the aperture of the body chamber and the visceral mass containing other vital organs.

Each chamber is partitioned from those adjacent by a sheet of calcareous material, the septum; the suture is formed where each septum is cemented to the outer shell. Suture patterns in Nautilus are simple.

Jet propulsion : Ink is ejected by hyponome which came through the ink sac within mantle cavity.

Cameral deposits : Within shell, there are endosiphuncle calcareous deposits, called cameral deposits. Only orthoconic and cyrtonic shells have these deposits.

Siphuncle tube is permeable have living material, arteries and veins. Septal neck is impermeable.

Ammonoidea

• They were originated in Devonian from Bactritoidea.

• Shell is planispirally coiled except in heteromorphs.

• Hyponome is ventral. Sutures are complex.

• Septal neck is short and prochoanitic.

• Shells have keel and lappets.

• Important for stratigraphic correlation of Mesozoic rocks.

• They became extinct on Cretaceous Tertiary mass extinction event.

These have been divided into 4 categories

Goniatitida 

These forms have goniatic sutures. Siphuncle is present on ventral side. They lived from devonian to permian.

Ceratitida

These forms have ceratitic sutures. Siphuncle is present on ventral side. They lived from devonian to triassic.

Clymeniida 

Siphuncle was present on dorsal side. They lived in upper devonian. Ex. Clymernia.

Ammonitida

These forms have complex ammonitic sutures. Siphuncle is present on ventral side. They lived from Triassic to cretaceous.

Suture patterns are important to study evolution.

Sutures Representation

To clarify, they represented graphically and drawn from venter to umbilical seam and hence to the dorsum of septa.

Ventral arrow point towards aperture and represented on lift side.

Dorsal side is represented on right side arrow.

Saddles are upwards and lobes are on downward side.

Types of sutures

Gonatitic sutures : These sutures have undivided saddles and lobes.

Ceratitic sutures : Lobes are frilled.

Ammonitic sutures : Both lobes and saddles are frilled.

Sutures were evolved from gonatitic to ceratitic and then  to ammonitic.

Evolution

Evolution pattern in ammonites was Iterative evolution. In this, ancestors give rise to new short living groups time to time which replaces other successively.

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