would seem that we have all gradations from separate openings, 

 near the outer end of the kidneys, openings near the peri- 

 cardial end of the kidneys, and double openings, so the genital 

 ducts may be connected with the kidneys by branchs and 

 be continued to the outside as well ; most of which arrangements 

 are present among members of the Protobranchia, it is still very 

 doubtful whether any significance can be attached to the rela- 

 tionship of the genital ducts and kidneys in different forms. 



CIRCULATORY SYSTEM. 



The animal is large enough to allow one to successfully inject 

 the chief vessels with starch or gelatin injecting masses, and 

 then by dissection and microscopic preparations to trace the dis- 

 tribution of the vessels of the different organs and to determine 

 quite definitely the course taken by the blood in its circulation. 



The heart is a typical, symmetrical lamellibranch heart with 

 two auricles and one ventricle (fig. n, 13 and 20) the latter 

 perforated by the intestine which enters it near one end and 

 leaves it near the other end. Dorsally the ventricle is prolonged 

 somewhat, posterior to the intestine, where the morphologically 

 anterior aorta is given off, and ventrally to a less extent it is 

 prolonged anterior to the intestine, where the much smaller 

 morphologically posterior aorta is given off. The walls of the 

 ventricle are of about even thickness throughout their extent, 

 and are quite smooth outside and inside. The auricles join the 

 ventricle on each side near its middle, are somewhat triangular 

 in shape, with the most acute angle receiving blood from the 

 gills and mantle, at a point dorsal to the adductor muscle, and 

 directly ventral to, but some distance from, the cartilage. 

 The opening of each auricle into the ventricle is near the middle 

 of the side of the auricle that lies next to the ventricle and 

 farthest away from the opening where the auricle receives its 

 blood. The muscles around the openings of the auricles into 

 the ventricle, and to a less extent around the openings through 

 which the auricles receive blood, are well developed and must 

 act as spinctors that tend to keep the blood from being regurg- 

 itated. The walls of the auricles, unlike those of the ventricles, 

 are roughened by pits that open into the cavities of the auricles. 

 These seem to be formed by the arrangement of bands of muscle 



35 



