these organs is distributed to the kidneys through systems of 

 capillary spaces. The branching of these vessels is not conspic- 

 uous on the surface of the kidneys, but is better seen by 

 cutting the kidneys open. That not all of the blood necessarily 

 traverses the capillary spaces of the kidneys is indicated by the 

 fact that injections of the systemic veins frequently fill the vein 

 that carries blood away from the kidneys as well as those lead- 

 ing to it. This is much more frequently the case when injecting 

 from the posterior surface of the adductor muscle than when 

 injecting from other places, and seems to be dependent upon a 

 direct connection between the vessel in question and the sinuses 

 on the anterior and ventral surface of the adductor muscle near 

 the dorsal ends of the kidneys. 



Of the blood that leaves the heart, only that which goes to the 

 mantle remains to be accounted for. This is collected and 

 returned directly to the heart (fig. 9, pv.). 



All of the blood that leaves the kidneys is conducted to the 

 gills. The blood from each kidney is collected into a sinus that 

 runs along the border of the kidnev that is applied to the adduc- 

 tor muscle. This sinus, which also seems to receive blood from 

 the sinuses on the anterior and ventral surface of the adductor 

 muscle, bends abruptly ventrally over the anterior end of the 

 kidney and is continued on the lower border of the suspensory 

 membrane of the gill (fig. 11, ba.) to the posterior end of the 

 gill, supplying the gill with branches throughout its length. 



The blood vessels of the gills have been described in connec- 

 tion with the structure of the gills, but for the sake of com- 

 pleteness the course of the blood through the gills will be traced 

 in this connection. 



Blood vessels leave the vessel that carries blood from the 

 kidney, opposite each of the inter-lamellar junctions of each of 

 the gills supported by the suspensory membrane. Each of these 

 branches is continued along the free border of the membrane 

 that forms the inter-lamellar junction (figs. 17 and 18, ba'.) 

 until it reaches the free edge of the lamella, the edge that is not 

 attached to the suspensory membrane. That is, if the branch 

 supplies an outer gill, it leaves the suspensory membrane along 

 the free border of an inter-lamellar junction and crosses over 

 to the free border of the outer lamella of this gill. Here the 



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