SG6 



HISTORY OF FISHES. 



the bivalved tribe of animals in every respect 

 inferior to those we have been describing. 

 Inferior in all their sensations ; inferior in 

 their powers of motion ; but particularly in- 



This instrument is of great use to such shell-fish as 

 conceal themselves in the mud or sand, which its struc- 

 ture is then peculiarly adapted for scooping out. The 

 cockle continually employs its foot for this purpose ; first 

 elongating it, directing its point downwards, and insin- 

 uating it deep into the sand, and next, turning up the 

 end, and forming it into a hook, by which, from the resis- 

 tance of the sand, it is fixed in its position, and then the 

 muscles which usually retract it are thrown into action, 

 and the whole shell is alternately raised and depressed, 

 moving on the foot as on a fulcrum. The effect of these 

 exertions is to drag the shell downwards. When the 

 animal is moderately active, these movements are repeated 

 two or three times in a minute. The apparent pro- 

 gress is at first but small ; the shell, which was raised on 

 its edge at the middle of the stroke, falling back on its 

 side at the end of it ; but when the shell is buried so far 

 as to be supported on its edge, it advances more rapidly, 

 sinking visibly at every stroke, till nothing but the 

 extremity of the tube can be perceived above the 

 sand. 



By a process exactly the inverse of this, that is, by 

 doubling up the foot, and pushing with it downwards 

 against the sand below, the shell may be again made to 

 rise by the same kind of efforts which before protruded 

 the foot. By this process of burrowing, the animal is 

 enabled quickly to retreat when danger presses, and when 

 this is past, it can, with equal facility, emerge from its 

 hiding-place. 



The Cardium can also advance at the bottom of the 

 sea along the surface of the soft earth, pressing back- 

 wards with its foot, as a boatman impels his boat onwards 

 by pushing with his pole against the ground in a con- 

 trary direction. It is, likewise, by a similar expedient, 

 that the Solen forces its way through the sand, expand- 

 ing the end of its foot into the form of a club. The 

 Tellina is remarkable for the quickness and agility with 

 which it can spring to considerable distances, by first 

 folding the foot into a small compass, and then suddenly 

 extending it, while the shell is at the same time closed 

 with a loud snap. 



The Pinna, or marine mussel, when inhabiting the 

 shores of tempestuous seas, is furnished, in addition, 

 with a singular apparatus for withstanding the fury of the 

 surge, and securing itself from dangerous collisions, 

 which might easily destroy the brittle texture of its shell. 

 The object of this apparatus is to prepare a great num- 

 ber of threads, which are fastened at various points to the 

 adjacent rooks, and then tightly drawn by the animal, 

 just as a ship is moored in a convenient station, to avoid 

 the buffeting of the storm. The foot of this bivalve is 

 cylindrical, and has, connected with its base, a round 

 tendon, of nearly the same length as itself, the office of 

 which is to retain all the threads in firm adhesion with 

 it, and concentrate their power, on one point. The 

 threads themselves are composed of a glutinous matter, 

 prepared by a particular organ. They are not spun by 

 being drawn out of the body like the threads of the silk- 

 worm, or of the spider, but they are cast in a mould, 

 where they harden, and acquire a' certain consistence 

 before they are employed. This mould is curiously 

 constructed ; there is a deep groove which passes along 

 the foot, from the root of the tendon to its other extrem- 

 ity, and the sides of this groove are formed so as to fold 

 and close over it, thereby converting it into a canal. 

 The glutinous secretion, which is poured into this canal, 

 dries into a solid thread ; and, when it has acquired 

 sufficient tenacity the foot is protruded, and the thread 

 it contains is applied to the object to which it is to be 



ferior in their system of animal genera- 

 tion. The snail tribe, as we saw, are her- 

 maphrodite, but require the assistance of each 

 other for fecundation ; all the bivalve tribe 

 are hermaphrodite in like manner, but they 

 require no assistance from each other towards 

 impregnation ; and a single mussel or oyster, 

 if there were on other in the world, would 

 quickly replenish the ocean. As the land- 

 snail, from its being best known, took the 

 lead in the former class, so the fresh-water 

 mussel, for the same reason, may take the 

 lead in this. The life and manners of such 

 as belong to the sea will be best displayed in 

 the comparison. 



The mussel, as is well known, whether 

 belonging to fresh or salt water, consists of 

 two equal shells, joined at the back by a 

 strong muscular ligament, that answers all 

 the purposes of a hinge. By the elastic con- 

 traction of these, the animal can open its shell 

 at pleasure, about a quarter of an inch from 

 each other. The fish is fixed to either shell 

 by four tendons, by means of which it shuts 

 them close, and keeps its body firm from being 

 crushed by any shock against the walls of its 

 own habitation. It is furnished, like all other 

 animals of -this kind, with vital organs, though 

 these are situated in a very extraordinary man- 

 ner. It has a mouth furnished with two 

 fleshy lips ; its intestine begins at the bottom 

 of the mouth, passes through the brain, and 

 makes a number of circumvolutions through 

 the liver; on leaving this organ, it goes on 

 straight into the heart, which it penetrates, 

 and ends in the anus : near which the lungs 

 are placed, and through which it breathes, 



fixed, its extremity being carefully attached to the solid 

 surface of that object. The canal of the foot is then 

 opened along its whole length, and the thread, which 

 adheres by its other extremity to the large tendon at the 

 base of the foot, is disengaged from the canal. Lastly, 

 the foot is retracted, and the same operation is repeated. 

 Thread after thread is thus formed, and applied in 

 different directions around the shell. Sometimes the 

 attempt fails, in consequence of some imperfection in 

 the thread ; but the animal, as if aware of the impor- 

 tance of ascertaining the strength of each thread, on 

 which its safety depends, tries every one of them as soon 

 as it has been fixed, by swinging itself round, so as to 

 put it fully on the stretch ; an action* which probably 

 also assists in elongating the thread. When once the 

 threads have been fixed, the animal does not appear to 

 have the power of cutting or breaking them off. The 

 liquid matter out of which they are formed, is so exceed- 

 ingly glutinous as to attach itself firmly to the smoothest 

 bodies. It is but slowly produced, for it appears that no 

 Pinna is capable of forming more than four, or at most 

 five threads, in the course of a day and night. The 

 threads which are formed in haste, when the animal is 

 disturbed in its operations, are more slender than those 

 which are constructed at its leisure. In Sicily, and 

 other parts of the Mediterranean, these threads have 

 been manufactured into gloves, and other articles, which 

 resemble silk. Abridged from Dr Rogcfs firidgewatcr 

 Treatise. 



