vessel is continued down the enlarged, modified filament that is 

 concerned in the formation of the inter-lamellar junction (fig. 

 17, ba".) giving out side branches through each of the inter- 

 filamenter junctions (as long as these are composed of tissue 

 that can carry blood vessels) and so supplies the various fila- 

 ments of the lamella. The blood thus distributed finds its way 

 around the margin of the gill through small blood spaces and is 

 continued up the other lamella of the gill, the blood of the small 

 filaments being gradually collected through the vessels of the 

 inter-filamenter junctions into the vessels of the large filaments, 

 (fig. 17, bv'.) and by these poured into a vessel that lies just 

 beneath the vessel that supplies the gill and runs parallel with 

 it (figs. II and 18. bv.). This vessel receives all of the blood 

 from both of the gills of the side, and carries it directly to the 

 corresponding auricle of the heart. Just before the vessel 

 empties into the heart it receives a rather large vessel from the 

 corresponding lobe of the mantle which returns the blood that 

 was sent to the mantle, back of the heart. 



To sum up the course of the circulation of the blood briefly, 

 it will be seen that of the blood that leaves the heart only that 

 which is sent to the mantle is returned to the heart after travers- 

 ing a single get of capillary spaces ; that a small portion of the 

 blood sent to the adductor muscle (that which is collected by 

 the sinuses on the antro-ventral portion of the muscle) may be 

 returned after traversing two sets of capillaries— those of the 

 adductor muscle and those of the gills ; and that the greater por- 

 tion is returned only after traversing three sets of capillaries— 

 those of the general system, those of the kidneys, and those of 



the gills. 



The reasons for this arrangement of the circulatory system 

 are at least in part not hard to explain. The blood which 

 passes to the mantle loses some of its nourishing materials, 

 but as the mantle lobes are thin and are bathed over such a 

 large portion of their surfaces by a current of water, in which 

 there is an abundance of dissolved oxygen, respiration, no 

 doubt, takes place direct, and the blood has no need to pass 

 through the gills to get a supply. Again the work of the 

 mantle is not of such an active nature as to load the blood with 

 nitrogenous wastes. It seems likely that the amount of nitro- 



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