800 



BIVALVES THEIR DIGESTIVE ORGANS. 



usually after a very short course, dilates into a stomach, the 

 sides of which are perforated by the large hepatic ducts, 

 coming from the liver, in the centre of which the stomach 

 is imbedded. The intestine is very various in length ; and 

 its convolutions, interwoven with the liver and ovaries, are 

 generally contained in great part in the foot. Having made 

 its convolutions, the intestine is directed towards the heart, 

 through the ventricle of which it commonly passes, and ends 

 on the posterior muscle by an opening which, in some 

 species, has a divided margin. This anus is situated be- 

 tween the lobes of the mantle, and opens into the tube 

 which lies alongside and above the respiratory siphon. The 

 liver has a follicular structure, and is distinguished among 

 the other viscera by its green colour.* 



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 mussel. 

 In this figure (Fig. 54), a is the mouth, into which a bristle 



Fig. 54 



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

 observe 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 respira- 

 tory organs, passing through the midst of the heart at c, and 

 opening at d above the posterior muscle closing the shells, 

 beneath the small tube of the cloak. This description ap- 

 plies generally to most bivalves, but in the oyster the rectum 



* See Garner in Mag. Nat. Hist. n. s. iii. 164. Proc. Zool. Soc. iii. pt. i. 

 p. 127. Zool. Trans, i. 272. Deshayes in Cyclop. Anat. andPhys. i. 696. 

 Mery in Hist, de 1'Acad. Roy. des Sc. 1710, p. 40. 



