30 WALTER K. FISHER, 



veiu-iiet of the kidney. The final peripheral ramifications of the visceral 

 artery pass into the dorsal wall of the kidney, where there are several 

 layers of these vessels, among which the kidney spaces, lined with 

 ciliated epithelium, form an intricately fenestrated structure. The 

 limits of this excreting portion of the right kidney are shown in Figs. 20 

 and 22. These vessels then open into the kidney vein (K.S) which 

 nearly encircles the body, on the inner side of the spindle muscle, 

 at its upper edge. From the kidney sinus, ten vessels, five on each 

 side, pierce the spindle muscle just under the shell and on reaching 

 the mantle divide into many branches (Fig. 19) which distribute the 

 blood for aeration close under the epithelium. 



As shown in this figure the mantle hood covering the mantle 

 cavity over the head, is highly vascular. Vessels are distributed to 

 it directly from the anterior (right) portion of the kidney mesh, and 

 also from a forward prolongation of the kidney vein. On the left 

 side this blood flows directly into the circumpalhal sinus as it turns 

 toward the heart, but in front, the joining with the sinus takes place 

 less directly, in a manner to be described shortly in connection with 

 the mantle circulation. 



In Fig. 21 a portion of the stomach and intestine is shown with 

 some of the branches of the visceral artery passing directly into the 

 vein-net of the kidney. Finer twigs of both the dorsal and ventral 

 branches of the visceral artery are drawn together, so as to show the 

 relation of the two (see also Fig. I). Not all the blood from the 

 visceral mass reaches the kidney so directly. Many of the smaller 

 arteries branch and rebranch to capillary fineness, particularly those 

 of the upper layer of the liver, and the blood passes into minute spaces 

 or capillary lacunae between the lobules of the liver. It finally col- 

 lects in more or less definite spaces between the dorsal surface of 

 the liver and the dorsal epithelium of the body, and thence passes 

 into the vein-net of the kidney. Hence the space or cavity in which 

 the alimentary canal and liver lie, must be regarded from an ana- 

 tomical standpoint at least, as a blood space, homologous with the 

 visceral cavity of the chitons. It should be remembered in this con- 

 nection that the cavity is so completely packed with liver tissue and 

 alimentary canal that only the minutest lacunae survive, and that in 

 point of fact, no sinus as such now exists. This primary body cavity 

 or pseudhaemal space is separated from the coelom, or secondary 

 body cavity below, in which is the gonad, by a thia epithelium, which 



