THE ANATOMY OF DORIS. 123 



This cavity is that of the renal organ. It is lined with a very 

 beautiful glandular tissue, and opens externally at the small 

 orifice above mentioned as situated close to the anus. The blood 

 from the network on the walls of this renal organ passes into the 

 tissue of the liver, and thus, with the rest of the blood of the 

 liver, is carried backwards by the great hepatic trunk vein to the 

 branchias. 



It will be observed that the renal organ, the liver, and, pro- 

 bably the ovarium, all receive a supply of venous, as well as of 

 arterial blood : the arterial blood is sent to them by the systemic 

 heart along the aorta, the venous by the second heart along a 

 system of vessels, which may fairly be said, as well as this heart 

 itself, to have a portal character, inasmuch as they convey blood 

 which has already arrived at the venous stage of the systemic 

 circulation. 



This is the first notice in the mollusca of a portal apparatus 

 which, in them, as in the lower fishes, subserves the renal and the 

 hepatic organs. 



At the root of the branchial crown there are, within the skin, 

 two concentric canals, the inner receives from the front the great 

 hepatic vein, and communicates with the outer by means of 

 channels, which run up the inner and down the outer side of the 

 branchial leaflets. The outer circle opens freely in front by a wide 

 short canal, into the posterior margin of the systemic auricle, on 

 the median line. This is the course of the branchial circula- 

 tion. 



It is, then, apparent that in these mollusks there is a triple 

 circulation. First, the systemic, in which the blood propelled along 

 arteries to the viscera and foot, is returned, with the exception of 

 that from the liver mass, to the heart, through the skin. There 

 it becomes partially aerated, the skin being provided with vibratile 

 cilia, and otherwise adapted as an instrument of respiration. 

 Second, the portal : in which venous blood from the system is 

 driven, by a special heart, to the renal and hepatic organs, and 

 probably to the ovarium, whence it escapes, doubly venous, with 

 the rest of the blood which has been supplied to these organs from 

 the aorta, and which is therefore only singly venous, to the 



