494 



CONCHIFERA. 



secretion being poured into the stomach itself through several large 



Fig. 248. 



Alimentary canal of the Oyster (Ostrea edullft). 



orifices, represented in 

 the figure. A very pecu- 

 liar arrangement exists 

 in the stomachs of many 

 genera, the digestive 

 cavity being prolonged 

 in one direction, so as 

 to form a lengthened 

 cascum, or blind sac- 

 culus, wherein is lodged 

 a cartilaginous styliform 

 body, the use of which 

 is not easy to conjecture, 

 although its office is no 

 doubt connected in some 

 way or other with the 

 preparation of the food. 

 The liver is proportion- 

 ately of large dimen- 

 sions, and is at once re- 

 cognized by its greenish 

 or, in some cases, dark chocolate colour ; it is entirely separable into 

 masses of secerning follicles loosely connected together by a delicate 

 cellulosity. The intestine varies considerably in extent, and, as a 

 necessary consequence, in the arrangement and number of its convolu- 

 tions. In the Oyster it is comparatively short, bending twice upon 

 itself, and winding around the stomach and adductor muscle (6, c, d,f) ; 

 its termination (</) projecting between the folds of the mantle upon the 

 opposite side of the body to that where the mouth is situated, and so 

 disposed that excrementitious matter is cast out beyond the influence of 

 the ciliary currents. In Pecten we have already noticed that it performs 

 sundry gyrations through the visceral mass, as well as about the muscle 

 that closes the shell (fig. 247, c, n, m) ; while in the Cockle-tribes it 

 even penetrates the base of the foot and winds extensively through 

 its muscular substance (fig. 253). In the greater number of the Con- 

 chifera, but not in the Oyster-tribe, there is a very remarkable circum- 

 stance connected with the course of the intestine, the object of which is 

 involved in obscurity : the rectum, at some distance from its termina- 

 tion, passes right through the centre of the ventricle of the heart, its 

 coats being tightly embraced by the muscular parietes of that viscus. 



(1294.) The position of the branchiaB in the Ostracean family has 

 been already described ; it now remains therefore to notice their inti- 

 mate structure, and the arrangement of the vessels connected with 

 respiration and the circulation of the blood. The branchial fringes are, 



