Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.2.2. External Features
Shape and size
The size varies from 5 to 10 cm. in length. The body is soft, unsegmented, bilaterally symmetrical and battened from side to side. It is sandwiched between the two valves of tie shell. When viewed from the side to: likely looks oval, with a blunt anterior end, the slackest part being in the middle of the body, near the dorsal side becoming gradually thinner toward the ventral edge.

Shell
The animal IS completely surrounded by a hard calcareous shut. It is brownish in colour.
Valves: The shell consists of two separate, equal and lateral pieces called values, covering the right and left sides of the body, respectively. The shill of a mussels made of two vales, is called bivalve as distinguished from the univalve shell of a snail, made of a single pied. In a smile flew, the shell is opal, the anterior and being rounded and the posterior mere pointed.
Hinge-ligament: Write two valves of tie shill are united tomato along the aerial id: in a straight bilge line by an external, browns tough, illicit - and note' calcareous or horny hinge-ligament made of conohlolin. The gape of the shell is ventral. The Elastic ligament draws the valve: together dorsally and causes them to gape ventrally.
Umbo: Dorsally, in front hinge ligament of the dingo and nearer the anterior dorsal pillar end, there is a whitish knob-like pert re swelling in each valve, called the umbo, which are the thickest and the oldest portion of the shell. It is the first part or the shell the develop in the late veliger larval stage. It is usually corroded by the action of carbonic and hurdle acids in water. Since entitle edge tie umbo is directs d anteriorly, it is possible to determine the right and left posterior
Lines of Growth: The outer surface of each shell valve presents a number of concentric lines around the umbo as centre and running parallel to the free large of the shell.
These are the lanes or benign of growth, representing intervals between successive growth stag's.

The shell increase in size by the deposition of new rings around the outer rim of the shell each year. Rings that are far apart indicate periods of rapid growth, owing to a plentiful food supply; periods of restricted growth are represented by close-fitting rings. The rings representing the dormant periods of the winter months are more conspicuous so that it is possible to estimate the age of a clam by these rings.
Hinge teeth
In Unio and Lamellidens, the inner surface of each valve possesses dorsally along the hinge line, small sharp ridges and teeth-like projections separated by grooves or sockets. These are known as hinge teeth. The teeth of one valve fit into the corresponding sockets of the other, thus holding the valves from slipping out of the position. They differ in number, shape and degree of development in different types of mussels and are entirely absent in Anodonta. The hinge-teeth close the shell with amazing precision and the margins of the shell articulate perfectly like the edges of a well-made locket. This precision is necessary because, if the shell valves did not join tightly, the clam would be much more vulnerable to its many enemies.
Muscle scars
The inner surface of each valve also bears characteristic markings or impressions, indicating the former attachment of muscles. Near each end, anteriorly and posteriorly, is a large and oval scar of the adductor muscle, that of the posterior muscle being larger than that of the anterior. Near the impression of the anterior adductor muscle are two small impressions, the dorsal and posterior one left by anterior retractor muscle, and the ventral and posterior by protractor muscle. A small impression of a posterior retractor muscle also lies dorsal to the impression of the posterior adductor muscle. Running inside and parallel to the free ventral margin of the valve, from one adductor impression to the other, is a fine groove or line, the pallial line, which marks the attachment of the muscle fibres from the edge of the mantle (retractor pallial muscles).
If a clam is molested, the foot at once withdraws in- side the shell, the two valves of which are slowly and lightly shut by the powerful anterior and posterior adductors, thus barring the entry of in-truders. The attempt to pull apart the two valves of the shell may not succeed unless a thin-bladed knife is first inserted through the gape of the shell to severe the large adductor muscles. The star¬fish, however, has a novel way of opening the shell valves. It circumvents the clam, attaches its tube feet to the shell valves and exerts a steady pull. Sooner or later, the adductor muscles of the clam become exhausted and relaxed so that the shell opens.