their- Food and Digestive Organs, 



221 



latter, we find a stylet or cylinder of firm gelatine lying loosely 

 detached, but of a size which seems more than equal to plug 

 up the passage. Of the use of this body, as well as of the 

 mode of its formation, Blainville acknowledges his total 



Ignorance 



To give you a clearer idea of the course of the alimentary 

 canal in this class of animals, I will copy, on a reduced scale, 

 Sir E. Home's figure of it, taken from the freshwater muscle. 

 (Ji^g, 37'), a is the mouth, into which a bristle 



In this figure 



o 



has been introduced, b the stomach, after which you will ob- 

 serve that the intestine makes five turns in the foot amidst the 

 ovary, and then, as rectum, runs posteriorly along the back 

 of the animal beneath the hinge and above the respiratory 

 organs, passing through the midst of the heart at c, and open- 

 ing at d above the posterior muscle closing the shells, beneath 

 the small tube of the cloak. This description applies gene- 

 rally to most bivalves, but in the oyster the rectum does not 

 pass through the heart. (Cams, Comp. Anat., trans, vol. ii. p. 7.) 

 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 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. 

 (Zool. Jou7'n., vol. i. p. 581.) Other bivalves are probably 

 nourished by similar animalcules ; for, when we reflect on 

 their apparently helpless and inert condition, hampered with 

 their shells, or even bound to the rock, we cannot but per- 

 ceive that they are all unfit for the capture of any other prey 

 than what floats about and within them. And how abun- 

 dantly is this furnished ! There are everywhere scattered on 



