HEAET AND RESPIRATORY SYSTEM. 



495 



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 terminate in large vessels enclosed in the 

 stems to which the fixed extremities of the vascular fringe are attached 

 (fig. 249, /, g, h, i) ; these communicate extensively with each other, 

 and, ultimately uniting in two principal trunks (e, k), pour the purified 

 blood derived from the whole branchial apparatus into the auricle of the 

 heart. 



(1295.) The heart in the Oyster (fig. 248, n, o) is situated in a cavity 

 between the folds of the intes- 

 tine and the adductor muscle, 

 in which position, from the 

 dark-purple colour which it 

 exhibits, it is at once distin- 

 guished. It consists, in the 

 species we are more particu- 

 larly describing, of two distinct 

 chambers an auricle and a 

 ventricle. The auricular cavity 

 (fig. 249, b), the walls of which 

 are extremely thin, and com- 

 posed of most delicate fasciculi 

 of muscular fibres, receives the 

 blood from the respiratory ap- 

 paratus, and by its contraction 

 transmits it through two inter- 

 mediate canals (c) into the 

 more muscular ventricle (d), 

 whence it is propelled through 

 the body by the ramifications 

 of the arterial system (n, o,p}. 



(1296.) The above description of the circulatory apparatus as it 

 exists in the Oyster is applicable in all essential points to every family 

 of Conchiferous Mollusca ; but there are important modifications in the 

 structure of the heart and arrangement of the blood-vessels, met with in 

 different genera, which now demand our attention. Most generally, in 

 consequence of the broad and dilated form of the animals, instead of a 

 single auricle, such as the Oyster has, there are two auricular cavities, 

 one appropriated to each pair of branchial Iamella3, and placed symme- 

 trically on the two sides of an elongated fusiform ventricle, into which 

 both the auricles empty themselves, till the course of the blood is 

 similar to what we have described above. 



(1297.) A still greater modification is found to exist in those species 



Heart and respiratory system of the Oyster. 



