322 FOSSIL OSTEEID^ OF NORTH AMERICA. 



vital waste and repair. It is likely tbat they are formed indirectly 

 from tlie nutritive matters which have been absorbed from the food 

 through the walls of the intestine and stomach ; in fact, thin sections 

 often show an abundance of similar corpuscular bodies in the tissues 

 immediately adjoining the intestinal walls, the presence of which in 

 such situations would seem to be most readily explained by the view 

 here suggested. Whatever may be the mode of their origin, their 

 structure and amoeboid characteristics would indicate that as they are 

 carried through the body of the animal by the blood current they take 

 an all-important part in the processes of growth and renewal of struct- 

 ure and the expulsion of worn-out or effete materials, both liquid and 

 gaseous. 



The vascular system of the oyster is not very easy to describe briefly 

 in an intelligible manner ; in fact, it is not yet clearly understood in all 

 of its details even by professed anatomists. The writer has, however, 

 traced the principal vessels and their connections with the heart, body, 

 and gills by a variety of methods, the results of which will be given 

 here in outline. 



The heart of the oyster is a much simpler organ than that found in 

 man or the higher animals. It consists of three principal divisions or 

 chambers, viz, a ventricle, partially divided in the middle line of the 

 body by a partition or septum, and two smaller inferior chambers, one 

 on either side, with darker walls than the ventricle. The relations of 

 these parts to each other are shown in Figs. 1 and 2, at ve and aw. The 

 three chambers of the heart are lodged in a crescent-shaped cavity just 

 in front of the adductor M and between the latter and the body-mass 

 in front, as may be seen in Figs. 1 and 2. This cavity is closed on either 

 side by a thin membrane, which is represented at c, in Fig. 1, detached 

 at its anterior border from the body-mass and thrown back over the ad- 

 ductor muscle. It contracts much inore slowly than the heart of 

 higher animals, and even much more slowly than that of snails or gas- 

 tropod mollusks. The normal number of beats of the heart of the oys- 

 ter in life probably does not much exceed twenty per minute, if its 

 pulsations are even so rapid as this. When fully distended the ven- 

 tricle nearly fills the crescent-shaped space in which it lies, but falls far 

 short of filling it when contracted. These two opposite conditions of 

 dilitation and contraction of the heart are represented in Figs. 1 and 2. 



The walls of the ventricle are very much thicker than those of the 

 auricles, and are mainly composed of muscular fibers, which interlace 

 with each other in various directions, and which contract and elongate 

 simultaneously, so as to increase and diminish the caijacity of the cav- 

 ity of the heart alternately, thus constituting a veritable living pump- 

 ing apparatus. This apparatus is rendered still more effective by rea- 

 son of the two valves which are interj^osed between the ventricle ve 

 and auricles au, the presence of which prevents the blood from flow- 

 ing back into the auricles from the ventricle when the latter contracts. 



