Anatomy of finfish and shellfish

Unit 13- Mollusca

13.4.2. External Features
Shape and size
The cuttle-fish has a fishy, bilaterally symmetrical and dorso-ventrally flattened body, which is shorter and flatter than that of Loligo, and tapers towards the posterior end. The anterior and posterior ends of the body in fact represent the dorsal and ventral ends due to much elongation of the dorso-ventral axis of the body. The average size is about 30 cm. The smallest cuttle-fish, belonging to the genus Idiosepius, measures about 15 mm. in length.
The body is divisible into an anterior prominent head and a posterior trunk, united by a constricted neck.
Head
The head bears a pair of large, highly developed eyes at the sides, and a mouth at the free extremity, surrounded by 5 pairs of tapering, muscular circumoral appendages. These are differentiated into 4 pairs of short and stout arms and one pair of long tentacles, retractile into large pits at their base. The tentacles are used in the capture of prey and in copulation. The bases of all but the ventral pair of arms are united by an interbranchial web of integument. The inner flat surface of each arm bears four longitudinal rows of suckers. Each sucker is a muscular, shallow cup with a narrow, horny rim and supported on a short, thick stalk. The suckers can be firmly applied to the body of the prey or to any other object by creating a partial vacuum inside.
In the male cuttle-fish, the left fourth or ventral arm is modified or hectocotyilsed to serve as an intrornitteut organ, by the sup preio fiikers in its basal part. In tentacles, the suckers are found only on their terminal expanded portions. The apex of each tentacle bears a curious small terminal pad.
Trunk
The rest of the body or trunk is elongated and shield-shaped, with its base directed anteriorly and the ahoral nd or the apex posteriorly. It is slightly convex above and flat below and borded by a narrow frill like fin on either lateral side. The fins are separated by a cleft at the aboral end and are used in leisurely locomotion.
Mantle
Trunk is covered by a thick, muscular mantle, enclosing on the ventral side a large mantle cavity, which contains the viscera. Towards the oral end, the free mantle edge, encircling the narrow neck, forms a rounded lobe above and a collar below.
Funnel
Below the head lies a large conical muscular tube, the siphon or the funnel, projecting beyond the neck. It opens externally by a narrow aperture, but internally by a wide aperture, into the mantle cavity. A pair of cartilaginous knobs on the mantle fits into corresponding sockets on the posterior ventral surface of the funnel. A typical molluscan foot is not present. It is represented, in part at least, by the siphonal apparatus. The old concept, that circumoral arms and tentacles correspond to the forefoot or epipodiof the Gastropoda, seems to be erroneous; they are probably true cephalic appendages. Therefore, the name Siphonopóda has been suggested, instead of Cephalopoda, for the class.
Skin
The living cuttle-fish undergoes frequent changes of colour. These are brought about by numerous, large, pigment cells, the chromatophores and iridocytes, which lie in the dermis of the skin. The chromatophores are of three kinds: reddish, yellowish-brown and orange. They can be detailed by the contraction of radiating muscle fibres attached to the cell walls at one end, and to the skin at the other. By alternate con¬traction and expansion of the chromatophores, blushes of different hues pass rapidly over the surface of the body. The chromatophores, are less on the ventral side, which is much paler than the dorsal. The iridocytes, lying beneath the chromatophores, are transparent cells with a reticulate struc¬ture. They defract the light, producing the characteristic iridescence of the skin. The colour changes are controlled by the central nevous system. Preserved specimens usually loose their natural colour.
Shell
The shell of the cuttle-fish is internal, lying embedded, in the upper side, completely enclosed in a sac of the mantle ad secreted by its epithelial lining. It is flat, broad and oval in shape, represented by phragmocone with a broader and rounded oral end, called pro-ostracum, and a narrow, pointed aboral end, called rostrum projecting into a spine.
The shell is entirely dead and composed of calcareous rather than horny matter. The hard and resistant shell provides rigidity to the trunk, like an endoskele¬ton. The calcareous matter is arranged in fine parallel layers, the septa or laminae, enclosing spaces containing fluid and gas, so that the light shell serves as a hydrostatic organ or float and, owing to its dorsal position, helps in maintaining the equilibrium of the body.

Fig. 2. Sepia. Shell

The shell of cuttle fish, or the cuttle-bone, is a familiar object of the seashore. It is rather soft and spongy. Being light, it floats in water, and during monsoon, these bones are drifted ashore in such great numbers that they have been named as the sea-fram. It is given as a bill sharpener as well as a source of calcium to caged birds.

Fig. 3. Sepia. A male specimen with the mantle cavity laid open.

Mantle Cavity
When the mantle is cut mid-ventrally by a longitudinal incision and reflected, the voluminous mantle cavity is exposed. It extends posteriorly up to the apex—of the trunk. The funnel is seen towards the oral end with its narrow external and wide internal Openings. Basally, on its ventral surface, the funnel bears a pair of cartilaginous depressions or sockets, called the called the funnel cartilages. Opposite them, on the internal surface of the rejected mantle folds, are seen a pair of oval cartilaginous knobs, the mantle cartilage. Similarly, the dorsal surface of neck carries the nuchal cartilage, which fits against the dorsal cartilage on the mantle.

Fig. 4. Sepia. A mantle cavity of female specimen. Visceral sac partly dissected

The mantle cavity communicates with the outside in two ways, through the wide opening around the neck and by means of the funnel. When the mantle cavity enlarges, the inhalant water is drawn into the mantle cavity by the wide opening between the mantle collar and the neck. When the mantle cavity contracts, an interlocking occurs between the mantle and funnel cartilages ventrally and the nuchal and dorsal cartilages dorsally, so that the collar space is tightly closed. Thus, exhalent water has to pass out through the funnel in the interior of which a Cap-like valve, opening outwards, allows the water to run out from the mantle cavity but not in the reverse direction.
The bulk of mantle cavity is occupied by the visceral dome, consisting of various internal organs. The digestive gland lies anteriorly, bordered on either side by the retractor muscles of head and funnel. The median rectum opens by the central anus at the base of the funnel. On either side of the rectum extends a renal sac, opening into the mantle cavity by an external renal aperture upon a renal papilla. On the left side only is the .genital aperture, also at the end of a papilla. A little posterior lie the two large plume-shaped cienidia, one on each side. Two large stellate ganglia lie one on each side on the mantle wall, where the neck meets the trunk. Posteriorly, the round ink-sac can be recognized by its metallic colour. its duct runs forward ventral to the rectum Sand opens into it dorsally close to the anus. In the male, the testis lies partly covered by the ink-sac. In the female the renal sac is hidden from view by a pair of nidamental glands and a pair of accessory nidamental glands. The genital duct (penis in the male, oviduct in the female) opens on the left side near the left renal aperture.
Locomotion
The cuttle-fish swims about gently by the undulating movements of the fins which are also used in directing the course of the animal. But, the most important movements are the swift, darting movements with the help of the funnel, caused by rhythmical contractions of the mantle. When the mantle relaxes, the mantle cavity is enlarged and water is taken in through the collar space around the neck. When the mantle contracts the mantle-collar tightly fits on the neck closing this opening, so that the water in the mantle cavity is shot violently out through the siphon like a jet, and animal is propelled rapidly through water in the opposite direction. The resulting backward movement is so sudden and rapid that the cuttle-fish seems to-vanish instantly. The siphon may also determine the course of direction; when it is pointed forward, the cuttle-fish darts backward, and when, it is pointed backward, the animal is driven forward. The quick reaction is made possible by giant fibres in nerves, over which impulses travel very rapidly as in giant fibres of the earthworm. Man has employed jet propulsion in rockets and jet aircraft.
Coelom
The coelom is represented by the viscero-pericardial coelorn and the cavities of renal sacs. The former is a larger bag-like cavity extending backwards and divided by a constriction into two parts. The anterior part or pericardium encloses the hearts and communicates by two reno-pericardial apertures with the cavities of renal sacs. The posterior part or gonocoel encloses the gonad.