BIVALVED SHELL-FISH. 



365 



serving like paddles to steer their course by. 

 When the weather is quite calm, and the 

 animal is pursued from below, it is then seen 

 expanding only a part of its sail, and rowing 

 with the rest : whenever it is interrupted, or 

 fears danger from above, it instantly furls the 

 sails, catches in all its oars, turns its shell 

 mouth downward, and instantly sinks to the 

 bottom. Sometimes also it is seen pumping 

 the water from its leaking hulk ; and, when 

 unfit for sailing, deserts its shell entirely. 

 The forsaken hulk is seen floating along, till 

 it dashes, by a kind of shipwreck, upon the 

 rocks or the shore. 



From the above description, I think we 

 may consider this animal rather as attempting 

 to save itself from the attacks of its destroyers, 

 than as rowing in pursuit of food. Certain 

 it is, that no creature of the deep has more 

 numerous or more powerful enemies. Its 

 shell is scarcely ever found in perfect preser- 

 vation ; but is generally seen to bear some 

 marks of hostile invasion. Its little arts, 

 therefore, upon the surface of the water may 

 have been given it for protection ; and it may 

 be thus endued with comparative swiftness, 

 to avoid the crab, the sea-scorpion, the trochus, 

 and all the slower predacious reptiles that 

 lurk for it at the bottom of the water. 



From this general view of snails, they ap- 

 pear to be a much more active animated tribe, 

 than from their figure one would at first con- 

 ceive. They seem to an inattentive spectator, 

 as mere inert masses of soft flesh, rather loaded 

 than covered with a shell, scarcely capable 

 of motion, and insensible to all the objects 

 around them. When viewed more closely, 

 they are found to be furnished with the organs 

 of life and sensation in tolerable perfection ; 

 they are defended with armour that is at once 

 both light and strong ; they are as active as 

 their necessities require ; and are possessed 

 of appetites more poignant than those of ani- 

 mals that seem much more perfectly formed. 

 In short, they are a fruitful industrious tribe; 

 furnished, like all other animals, with the 

 powers of escape and invasion ; they have 

 their pursuits and their enmities ; and, of all 

 creatures of the deep, they have most to fear 

 from each other. 



CHAP. VI. 



OF BIVALVED SHELL-FISH, OR SHELLS OF 

 THE OYSTER KIND. 



IT may seem whimsical to make a distinc- 

 tion between the animal perfections of turbin- 



* The Oyster, the Mussel, and the Cockle /The 





ated and bivalved shell-fish ; or to grant a 

 degree of superiority to the snail above the 

 oyster. Yet this distinction strongly and ap- 

 parently obtains in nature ; and we shall find 



Mollusca which inhabit bivalved shells, such as the 

 Oyster, the Mussel, and the Cockle, are all acephalous ; 

 that is, destitute of a head. The two valves of the shell 

 are united at the back by a hinge-joint, often very 

 artificially constructed, having teeth that lock into each 

 ther; and the mechanism of this articulation varies 

 much in different species. The hinge is secured by a 

 substance of great strength. 



During the life of the animal, the usual and natural 

 state of its shell is that of being kept open for a little 

 distance, so as to allow of the ingress and egress of the 

 water necessary for its nourishment and respiration ; but, 

 as a security against danger, it was necessary to furnish 

 the animal with the means of rapidly closing the shell, 

 and retaining the valves in a closed state. These ac- 

 tions being only occasional, yet requiring considerable 

 force, are effected by a muscular power, for which pur- 

 pose sometimes one, sometimes two, or even a greater 

 number of strong muscles are placed between the valves, 

 their fibres passing directly across from the inner surface 

 of the one to that of the other, and firmly attached to 

 both. They are named, from their office of bringing 

 the valves towards each other, the adductor muscles. 



The simple actions of opening and closing the valves, 

 are capable of being converted into a means of retreat- 

 ing from danger, or of removing to a more commodious 

 situation, in the case of those bivalves which are not 

 actually attached to rocks or other fixed bodies. Dique- 

 marc long ago observed, that even the oyster has some 

 power of locomotion, by suddenly closing its shell, and 

 thereby expelling the contained water with a degree of 

 force, which, by the reaction of the fluid in the opposite 

 direction, gives a sensible impulse to the heavy mass. 

 He notices the singular fact, that oysters which are at- 

 tached to rocks occasionally left dry by the retreat of the 

 tide, always retain within their shells a quantity of water 

 sufficient for respiration, and that they keep the valves 

 closed till the return of the tide ; whereas, those oysters 

 which are taken from greater depths, where the water 

 never leaves them, and are afterwards removed to situa- 

 tions where they are exposed to these vicissitudes, of 

 which they have had no previous experience, improvi- 

 dently open their shells after the sea has left them ; 

 and, by allowing the water to escape, soon perish. 



Many bivalved mollusca are provided with an instru- 

 ment shaped like a leg and foot, which they employ ex- 

 tensively for progressive motion. In the cardium, or 

 cockle, this organ is composed of a mass of muscular 

 fibres, interwoven together in a very complex manner, 

 and which may be compared to the muscular structure 

 of the human tongue ; the effect in both is the same, 

 namely, the conferring a power of motion in all possible 

 ways ; thus it may be readily protruded, retracted, or 

 inflected at every point. The solen, or razor-shell fish, 

 has a foot of a cylindrical shape, tapering at the end, and 

 much more resembling in its form a tongue than a foot. 

 In some bivalves the dilatation of the foot is effected by 

 a curious hydraulic mechanism: the interior of the organ 

 is formed of a spongy texture, capable of receiving a con- 

 siderable quantity of water, which the animal has the 

 power of injecting into it, and of thus increasing its 

 dimensions. 



The foot of the Mytilus edulis, or common mussel, can 

 be advanced to the distance of two inches from the shell, 

 and applied to any fixed body within that range. By 

 attaching the point to such body, and retracting the foot, 

 this animal drags its shell towards it, and by repeating 

 the operation successively on other points of the fixed 

 object, continues slowly to advance. 



