62 TESTACEA. 



it detached entire from the spot of adhesion, when its inhabitant always 



perishes. 



If free of decortication, and the tenant vigorous, the shell receives 

 occasional accessions, sometimes advancing visibly in confinement. The 

 curious internal and external structure of some, which it is so interesting 

 to behold, is understood to result from the peculiar organization of their 

 respective tenants — rings, spirals, flutings, cavities, or projections. The 

 structure of shells has been admirably illustrated of late by Dr Carpenter, 

 a learned physiologist, celebrated for the high quality of his writings, 

 and some other intelligent observers. 



From age or circumstances the shell is incessantly undergoing pro- 

 gressive modification. The change is not great or very conspicuous in 

 the mussel ; but the disproportion between the animal and the shell of 

 the oyster is augmenting daily : and in older specimens it is very great. 

 At first the shell is comparatively thin and delicate, and perhaps the 

 animal preponderates. At length the shell becomes clumsy, large, and 

 ponderous, weighing many times as much as its tenant ; while, further, 

 the sUo-ht elevation of the upper valve seems to have no influence over it. 

 The size and quality of all such productions are very dependent on 

 their site. Hence, while some increase and nudtiply rapidly in certain 

 places, others may remain stationary and scanty — mere dwarfs in com- 

 parison. Nevertheless, this is one branch of their history insufficiently 



investigated. 



It is from mud that the chief pabulum of the bivalves is derived. 

 Various animals, of very different genera are, besides, dependent on the 

 same source of aliment. I have not seen anything but muddy matter, 

 or muddy intermixtures, in the stomach. 



Certain sensations of the bivalves may be acute, but their instincts 

 ;ire imperfectly displayed, and their faculties, farther than opening and 

 closing their habitation, together with the application of the hyssus to use, 

 hardly demonstrated : unequivocal symptoms show how sensible they are 

 of dwelling in a grateful medium. Nothing is so essential to salubrity as 

 repeated replenishment of their vessels with recent water. Though 

 closely conliued by their shells, the bivalves demonstrate the impression 



