Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.3.2. External Features

Shape and size
The squid has a tapering somewhat torpedo-shaped or spindle-like body, hencethe nickname "sea-arrow".
Unlike other animals, the long axis of the body is dorso-ventral so that when the animal swims, the morphologically ventral surface is anterior, the dorsal surface is posterior, the anterior surface is dorsal and the posterior surface is ventral.
The average length for an adult squid is about 30 cm. but some species are only 25 cm. long.

Fig. 1- Loligo vulgaris Dorsal view

The largest cephalopods are the giant squids, Architeuthis, of the North Atlantic, one of which attained a total body length of 16 meters. The tentacles alone were 11 meters long and the body circumference 3-6 meters. A giant squid, captured in the Atlantic by the French battle¬ship Alecton, in 1860, was 15 meters long and weighed nearly 2 tons. They possess arms as large as man's legs and suckers as big as tea-cups. in another case, two separate tentacles, each about 12'6 meters long were found. Naturally, they must have come from a much larger specimen.
They have the distinction of being not only the largest molluscs, but also the largest invertebrate animals Since it is limited to the deep sea, it is rarely seen by man and hence is not well known. Most accounts of giant sea- serpents, though never been caught, are probably based on sudden and un expected glimpses of long writhing arms of a giant squid on the surface of the sea The sperm whales feed on these giant squids They said to engage in battles with whales whose dead bodies have been found bearing scars made by the suckers on the arms of a giant squid and their tongues eaten by the sharp beak of the squid.
Body
The body is divisible into an anterior head region and a posterior tapering visceral hump, jointed together by a narrow neck.

Fig. 2- Loligo vulgaris Ventral view

Head
The head is distinct, though small, and bears a pair of large eyes and the central mouth surrounded by 10 fleshy arms arranged in a ring. Two powerful horny jaws, like an inverted parrot's beak, protrude from the mouth, the ventral jaw overlapping the dorsal.
The foot is modified into the funnel and the 10 arms surrounding the mouth at the end of the head, 8 arms are short, stumpy and non-retractile and 2 are long, slender retractile tentacles, used in capturing prey. The inner surface of each arm bears pedicellate and cup-like suckers decreasing in size from the base to the tip of the arms. The suckers give the animal a good grip on anything around which it wraps the arms.
Visceral hump
It is long and pointed and bears two conical fins dorso-laterally towards the tail end, by means of which the animal swims forwards.
Mantle
The mantle forms the thick, muscular and protec¬tive envelope, enclos¬ing the visceral hump and the mantle cavity. The conical projections of the mantle, one on each side of the animal, form the fins. Ventrally, the free mantle edge forms a loose collar around the neck region, thus leaving a circular opening, through which water enters the mantle cavity. A conical mascular tube projecting beyond the collar, beneath the head, is the siphon or funnel, through which the water of mantle cavity is expelled. The funnel is the true representative of the molluscan foot. The collar articulates with the funnel and the visceral mass by three interlocking surfaces. The visceral mass occupies most of the space in the mantle cavity.

Fig. 3- Loligo Mantle cavity of the male dorsal view

Locomotion
The mantle and the funnel form the chief locomotory organs. The customary mode of the locomotion is slow swim¬ming by the undulating movements of the fins, during which the arms are closely extended in front to serve for steering. But, when the animal is excited, the mantle collar closes tightly around the neck and the water is forcibly ejected through the siphon, so that the animal is propelled in the opposite direction like a rocket or torpedo, after the principal of jet propulsion.
The rocket-like tapering body enables it to dart through water with lightning-like speed. The siphon is the chief steering organ; if it is directed forward, the jet of water passed through it propels the animal backward; if siphon is directed back ward, the animal is propelled forward. Squids attain the greatest swimming speeds among aquatic inver teberates Inspite of its rapid swimming, squid is often caught by large fish and some whales
Shell
The shell is internal, vestigial and embedded under the mantle mid-dorsally, extending from the edge of collar to the posterior end of the trunk. It is thin, light, nearly transparent, and feather-shaped horny plate with a stiffening rib down the centre on one face. It is called the pen or gladius because of its fancied resemblance to an old fashioned besides, a cartilage case surrounds the brain: a nuchal cartilage supports the neck and similiar cartilage support the siphon and the fins. The cartilage is remarkably similar to that of vertebrates and shows an instance of the convergent evolution.