32 SHELLS AND SHELL-FISH. PART I. 



shells, are provided with a thick tuft of silky hairs, 

 called a byssus, which issues from one side only of the 

 shell, the other end being firmly attached to m.arine 

 bodies. In both these families, however, there is still 

 some slight locomotive power left; for, although the 

 animal does not, and probably cannot, quit the spot by 

 disengaging its byssus, and mooring itself to another 

 station, still it is not rendered absolutely motionless : 

 the waves can move the shell backwards and forwards, 

 and the fish may be compared to a horse or other 

 animal fastened by a very short rope, which, neverthe- 

 less, permits him to move his body and limbs. In the 

 Pholas, we have the next step towards an absolute ex- 

 tinction of the power of moving. These shell-fish have 

 no byssus, nor are they affixed by any other method ; 

 yet, soon after birth, they perforate the substance upon 

 which they have been deposited by the parent, and ex- 

 cavate for themselves deep burrows, or caves, exactly of 

 the circumference of their own shells ; and in these 

 hollows they take up their permanent residence. It is 

 not absolutely ascertained, indeed, that these boring 

 Testacea cannot quit their first habitation for another ; 

 but there are many reasons to authorise the supposition 

 that they live and die in the same which they excavated 

 when young, and which they have the power of en- 

 larging as they increase in size. A\^ithin such a narrow 

 compass, it is obvious that the Pholas is more a prisoner 

 than the Pinna or the Terehratula, because the walls 

 of its habitation keep it immoveable, so that even the 

 agitation of the waves cannot give it that undulating 

 motion enjoyed by the byssiferous families. Finally, 

 we come to such as are absolutely cemented by one of 

 their valves to rocks or stones^ or to each other : many 

 of the oysters are of this description ; but the most con- 

 spicuous are the different species of Chama and spon- 

 dyles ; several of which have the entire surface of one 

 valve fastened by a calcareous deposition to rocks, pieces 

 of floating timber, and to the bottoms of ships. It is, 

 consequently, in these families, and in the worm-shells. 



