()2 



of the two arms of the sinus arc (•(iiiliiiii(»iis with and receive Ijlood hom the post- 

 erior vena cava. Each arm opens into the ventro-internal portion of the bran- 

 chial heart which hes just external to the dorsal third of each aim. The median 

 portion of the sinus is bounded below and behind by the angle of the gut 

 formed by the rectum and intestine, in h'ont by tlie ink sac, and above by the 

 pancreas. The remainder of the sinus lies undei- the pancreas , the heart , the 

 digestive sacs and , in the female . above the nidamental glands , or in the male , 

 just above the pallial chamber. The intestine ijasses between the two arms of 

 the sinus as they turn inward to unite with each other and with the anterior 

 vena cava. The sinus passes through the nephridial sac and is only attached to 

 the nephridial walls by its ends and by a narrow band of fascia along the upper 

 surface of the ai-ms. Paussek has described a pair of tubular, longitudinal cavities 

 in the mesoderm of the embryo from which the coelom and the nephridium 

 arise. Each tubular cavity soon after its appearance becomes differentiated into a 

 thin- walled doisal portion wliich tbrms the coelom and a narmw thicker walled 

 ventral portion which communicates with the exterior and becomes the ne- 

 phridium. This nephridial tube originally has no connection with its mate or 

 with the venous system l)ut as the embryo develops each nephridial cavity 

 enlarges so that its walls become moulded over the surface of the pancreas, the 

 intestine, and the veins. The two nephridial cavities become confluent by the 

 absorption of their opposed walls. In the adult the nephi'idial sac has the 

 shape of a retort whose neck is vertically flattened and is l)ilobed. The body of 

 the flask occupies the greater i)Ortion of the visceral mass between the heart 

 and the digestive sacs behind , and the liver and the ink sac in ft'ont . and 

 contains the pancreas , the intestine , and a part of the nephridial sinus. The 

 neck , — conceiving the retort to be inverted . — is directed backward beneath the 

 heart and digestive sacs, and in the female between them and the nidamental 

 glands. It contains the arms of the nephridial sinus which are separated dorsally 

 by what remains of the original sei)tum between the nephridial cavities. Conical 

 tubular canals lead from the t-oelom to the nephridium passing obliquely foi- a 

 short distance in the wall between the sac and the i)allial chamber. The canals 

 pass into the nephridium just above the branchial veins and at the junction of 

 the flask with the neck. By this arrangement the innei- wall of the canal fornis 

 a valve which prevents the escape of fluid from the nephridium into the coelom. 

 The nephridial sac opens upon the lower surface through two papillae, which lie 

 one on either side of the base of the rectum. The lining of the nephridial cavity 

 is reflected over all the structures within the cavity and . except where it covers 

 the venous vessels, is a ciliated pavement epithelium. Upon the sinus and the veins 



