28 MOLLUSC A. 



Pecten, the scallop, gives the easiest and best results, 

 besides being essentially typical in gill structure. 



The heart is really quite a powerful organ, and acts 

 with considerable power, though rather slowly. Its 

 contraction forces the blood through the arteries, and 

 from thence the same pressure causes this fluid to 

 continue its motion, when collected, into the branchial 

 capillaries and spongy masses onward into the veins 

 on its return to the heart. 



The muscles which do the pumping are to be seen 

 by opening the heart. They cross each other in the 

 interior of the ventricle in every direction, near the 

 walls and along the centre, and in the auricles they 

 have a similar distribution, though not so powerful. 

 The auricles and ventricle are furnished with valves, 

 which prevent the return of the blood towards the 

 veins, so that a self-acting pump is formed of three 

 compartments. The two auricles acting as feeders to 

 the ventricle, and the ventricle as the forcing agent 

 from which it is again distributed over the body. 



The generative organs (Figs. 12, 15, w) are immedi- 

 ately under the mantle, and envelop the viscera all over 

 the body, forming, when gorged with eggs or spermatozoa, 

 a thick, white coating of tumid-looking branches, termi- 

 nating in little, round sacs, or grape-like bodies, and are 

 a marked characteristic of the external aspect of the body 

 at certain seasons of the year. Upon examination, these 

 are found to be filled with httle granules, which are the 

 eggs. The sexes are said to be distinct by some investi- 

 gators ; by others it is affirmed that both eggs and sperma- 

 tozoa are elaborated in the same canals, and that the oyster 

 is truly a hermaphrodite. This question is not decided, 



