302 BIVALVES THEIR FOOD. 



mollusks, being found also in some Gasteropods, as, e. g. 

 in certain species of Strombus, in Trochus turritus, and in 

 a species of Murex.* 



Our knowledge of the food of bivalves may be considered 

 as almost entirely conjectural. It seems, however, to have 

 been ascertained, that oysters feed upon infusory vegetables 

 and animalcules ; and, as it has been asserted, that while 

 various species of these are beneficial, others are actually 

 injurious, it seems to follow that oysters must be able to 

 distinguish and reject the latter ; and the organs of discri- 

 mination are undoubtedly the sensitive labial palps that 

 guard the oral aperture. The green colour which oysters 

 acquire in their parks of brackish water, is owing to the 

 colour of the animalcules that furnish them their food ; and 

 hence M. Gaillon has remarked that other animalcules com- 

 municate to the oysters a colour similar to their own, tinting 

 them or brown, or grey, or yellowish, f 



Other bivalves are probably nourished by similar animal- 

 cules ; for, when we reflect on their apparently helpless and 

 inert condition, hampered with their shells, or even bound to 

 the rock, we cannot but perceive that they are all unfit for 

 the capture of any other prey than what floats about and 

 within them. And how abundantly is this furnished ! There 

 are everywhere scattered on the bed of the wide ocean 

 extensive beds of oysters, clams, mussels, &c., containing 

 millions of individuals, which are hourly devouring, each 

 of them, crowds of animalcules (embracing in the term the 

 infusory, microscopic, crustaceous and gelatinous medusae), 

 which, from their vast numbers and rapid reproduction, 

 never fail them. At some seasons of the year I have seen 

 the waters of our shores literally in a move with Entomos- 

 traca ; and I am fully satisfied that, when Scoresby calcu- 

 lated a cubical mile to contain 23,888,000,000,000,000, he 

 was not exaggerating the actual fact.J In one family of 



* Edinb. New Phil. Journ. vii. 231. 



f Edinb. New Phil. Journ. iv. 196. The excrement of oysters has 

 given rise to a curious simile ; " And though some count a jesting lie to be 

 like the dirt of oysters, 'which (they say) never stains, yet is it a sin in 

 earnest." FULLER'S Holy and Profane State, p. 379. 



$ " The number of Medusae in the olive-green sea was found to be im- 

 mense. They were about one fourth of an inch asunder. In this pro- 

 portion, a cubic inch of water must contain 64 ; a cubic foot, 110,592 ; a 

 cubic fathom, 23,887,872 ; and a cubical mile about 23,888,000,000,000,000 ! 

 From soundings made in the situation where these animals were found, 

 it is probable the sea is upwards of a mile in depth ; but whether these 

 substances occupy the whole depth is uncertain. Provided, however, 

 the depth to which they extend be but 250 fathoms, the above immense 

 number of one species may occur in a space of two miles square. It may 



