CIRCULATORY SYSTEM OF THE GIANT SCALLOP. 233 



(as long as these are composed of tissue that can carry blood 

 vessels) 1 and so supplies the various filaments 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-filamentar junctions into 

 the vessels of the large filaments (Fig. 7, &/), and by these poured 

 into a vessel that lies just beneath the vessel that supplies the 

 gill and runs parallel with it (Fig. i, h>). This vessel receives all 

 of the blood from both of the gills of the side, and carries it di- 

 rectly 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 set of capillary spaces ; that a small portion of the 

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

 the sinuses on the antero-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 ot 

 the gills. 



The reasons for this arrangement of the circulatory system 

 are at least in part not hard to find. 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 nitrogenous waste 

 in the blood that has traversed the mantle is so small that it 



'The inter-filamentar junctions near the free margins of the gills are composed of 

 cilia only. 



