Cephalopods are advanced molluscs in which foot is modified as tentacles and oral arms attached on the anterior side of the head. Sepia is a marine cephalopod found up to 3000 metre depth. They are predators and feed on fish, crabs, shrimps and prawns that they seize with prehensile tentacles and oral arms which are supplied with suckers. Sepia can also change colour and some are bioluminescent.
Body is divided into head and trunk. Head bears 8 oralarms and two tentacles, two large eyes and a pair of olfactorypits behind the eyes. Trunk has a visceral hump that carries internal shell called cuttlebone. Between the neck and trunk there is collar through which water enters into the mantle cavity. One pair of lateral fins help in swimming.
Visceral cavity can be opened by cutting mantle. There are three paired cartilages, namely, nuchal cartilages, mantle cartilages and funnel cartilages, which can interlock to close the collar opening. A funnel lies on the ventral side of the neck, through which exhalent current of water is expelled in the form of jet. Visceralmass lies on the posterior side of the mantle cavity and intestine and kidneys open at the base of funnel. One pair of large bipectinate ctenidia lay on either side of the visceral mass.
Mouth has circular fleshy lips and a pair of sharp jaws for tearing the prey into small pieces which are swallowed through the pharynx that lies in a small buccal mass. Radula is rudimentary in cephalopods. Two pairs of salivaryglands lie on either side of oesophagus and a salivary duct releases poisonous saliva at the base of jaws. Saliva also contains mucous and protein digesting enzymes. Oesophagus leads into a gizzard-like stomach that has a caecum or pyloric sac with spiral folds attached on its side. Stomach churns the food and crushes exoskeleton of arthropods on which Sepia feeds. Digestive gland has two sections, namely, a bilobed brownish liver and a cream coloured pancreaticappendage that surrounds the stomach. Ducts of digestive glands are called hepatopancreatic ducts that open into the pyloric sac of stomach. There is no intracellular digestion in the liver of cephalopods. Intestine is short and opens at the base of funnel. The inkgland also opens by a duct at the base of rectum. A cloud of ink released from ink gland helps the animal to camouflage from predators. Liver also serves to assimilate and store food.
One pair of bipectinate gills are the respiratory organs of Sepia. Water is inhaled through collar opening and expelled through the funnel opening. Inhalentphase is carried out by the longitudinal radialmuscles which contract to increase the volume of mantle cavity. Then the three pairs of cartilages interlock to close the collar opening. For exhalent phase,circularmuscles contract and radial muscles relax to increase pressure of water inside the mantle cavity. The collar opening being closed, water is forced through the funnel in the form of a jet. Haemocyanin is the respiratory pigment in cephalopods.
Circulatory system is closed type in cephalopods and there is complete separation of arterial blood from venous blood. There is one systemicheart or arterial heart and two venoushearts or branchial hearts at the base of gills. Systemic heart is enclosed in a pericardial cavity and consists of two auricles and one median ventricle. Auricles receive oxygenated blood from gills through the efferent branchial vessels and pour it into the ventricle. Ventricle pumps blood into the anterior or cephalicaorta that supplies blood to the head region and posterioraorta that takes blood to the visceral organs.
The venous hearts are located at the base of gills and receive blood from the anterior side through the precava and from the posterior side through the abdominalveins and pallialveins. This deoxygenated blood is pumped by the venous hearts to the gills through the afferent branchial vessel. There are no sinuses and lacunae in cephalopods. Blood contains haemocyanin dissolved in plasma and amoebocytes.