380 



CONCHIFERA. 



the shell (Jig. 176, o, w, m) ; while in the cockle tribes it even 

 penetrates the base of the foot, and winds extensively through its 

 muscular substance (fig. 182). In the greater number of the 

 Conchifera, but not in the Oyster tribe, there is a very remarkable 

 circumstance connected with the course of the intestine, the object 

 of which is involved in obscurity ; the rectum, at some distance from 

 its termination, passes right through the centre of the ventricle of 

 the heart, its coats being tightly embraced by the muscular parietes 

 of that viscus. 



(414.) The position of the branchiae 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 of course essentially vascular in their composition ; being, in 

 fact, made up of innumerable delicate parallel vessels enclosed in 

 cellular tissue of extreme delicacy, and exposing a very extensive 

 surface to the influence of the respired medium. The countless 

 branchial canals through which the blood is thus distributed termi- 

 nate in large vessels enclosed in the stems to which the fixed extremi- 

 ties of the vascular fringe are attached (fig. 178,/, g, A, i) ; these 

 communicate extensively with each other, and, ultimately uniting in 

 two principal trunks (e, &), pour the purified blood derived from 

 the whole branchial apparatus into the auricle of the heart. 



The heart in the Oyster Figf 178> 



(fig. 177, n-> o) is situat- 

 ed in a cavity between the 

 folds of the intestine and 

 the adductor muscle ; in 

 which position, from the 

 dark purple colour which it 

 exhibits, it is at once dis- 

 tinguished. It consists, in 

 the species we are more 

 particularly describing, of 

 two distinct chambers, an 

 auricle and a ventricle. The 

 auricular cavity (fig. 178, 

 b), the walls of which are 

 extremely thin, and com- 

 posed of most delicate fas- 

 ciculi of muscular fibres, re- 



