TESTACEA. 59 



approximated to the Ascidia, none inhabit this country detached from 

 the shell. 



The genera of both univalves and bivalves are very numerous : great 

 variety is seen everywhere in the testaceous covering. 



The species are almost infinite : yet, let us look within ; most of the 

 tenants, on cursory inspection, bear a common resemblance. The only 

 difference seems to be in size. Thus it may be reasonably concluded that 

 the true organic structure has passed overlooked : it has not been hitherto 

 observed. 



Many of the univalves are terrestrial ; they feed on various vegetable 

 products, often so ravenously as to pi-ove a real scourge to cultivators of 

 the soil. All the bivalves dwell in the waters, whether fresh or salt : 

 they sink among the mud or burrow in the sand. 



I do not know that the animals of the bivalves feed on solid matter : 

 they are not seen to consume visible substances, but their whole suste- 

 nance seems to be eliminated from mud and muddy solutions. Where 

 those are abundant the stomach is full, the body enlarged, and the ani- 

 mals, of some kinds at least, remain plump and vigorous. Gratification 

 is evidently derived from the presence of muddy solutions, and nutrition 

 follows their absorption. 



The greater proportion dwell in deeper water, but some are abso- 

 lutely littoral, dwelling at or above low-water mark. 



Some of the bivalve tribe, though not those of Britain, are the 

 largest of the Testacea. Although some of the univalves of distant 

 shores are of gigantic size, the bivalves far exceed them. Little, how- 

 ever, is accurately known of the precise natures of either. 



Noae of the bivalves familiar to us are adapted by configuration to 

 speedy motion. In general their progression is slow and laboured. Those 

 that can advance have an organ, performing somewhat the office of a 

 foot, protruding from the shell ; some spring through the way by the 

 sudden closure of the two shells when opened, but many seem to 

 have no farther power whatever than merely elevating the upper, as 

 the shell rests or may be affixed to some solid substance, or is incorpo- 

 rated with it by gradual adhesion. There are some that change their 



