THE MOLLUSC A 15 



and auricle of the same side (topographically the right side) are 

 also atrophied or have disappeared (Fig. 55). 



Almost all the venous blood that passes to the ctenidia tra- 

 verses the kidneys, so that there is a renal portal system. The 

 renal sacs are, in fact, irrigated by conduits which lead to the 

 afferent branchial veins, and these conduits may traverse the 

 kidneys, as in Cephalopods (Fig. 273), or may surround them, as 

 in Septibranchs (Fig. 210). Consequently the blood passing through 

 the ctenidia is devoid of the products of excretion. 



The surface of the excretory sac which forms the kidney may 

 be greatly increased by folds, by the formation of caeca, etc. Its 

 walls are glandular for a greater or less part of their extent, and 

 consist of an excretory epithelium in the cells of which the 

 nitrogenous products of metabolism are accumulated. These 

 products are ejected in the solid or liquid form, and vary from one 

 group to another as regards their chemical constitution. They 

 consist essentially of guanin in the Cephalopods, of uric acid in 

 Gastropods, except in Cydostoma, where they consist of urea, as is 

 also the case in Lamellibranchs, in which group uric acid is not 

 normally found. The external water does not penetrate into the 

 kidney, nor, a fortiori, does it enter the pericardium. It has, 

 however, been established that water may occasionally enter the 

 kidney of certain Heteropods and of Styliger, an Opisthobranch of 

 the family Hermaeidae. 



The glandular part of the kidney is not the only region in 

 which a glandular epithelium may be present. The epithelial 

 lining of the pericardium may, in various groups, be specialised to 

 form a pericardial gland (Grobben) whose excretion is more acid 

 than that of the kidney properly so called. Such a gland may be 

 seen on the surfaces of the auricles or in the ramifications of the 

 pericardium in Gastropods, Lamellibranchs (Fig. 212), and in 

 Cephalopods (Fig. 273). This glandular region has a blood supply 

 analogous to that of the kidney, and one may even see, in Nautilus, 

 the renal epithelium and that of the pericardial gland developed at 

 the same level on the same afferent branchial vessel, the one on the 

 one side, the other on the other side. The pericardial gland 

 eliminates the waste products which are excreted by the Malpighian 

 glomeruli of the vertebrate kidney ; the molluscan kidney, properly 

 so called, deals, on the other hand, with the same products of 

 excretion as the tubuli contorti. Certain liver cells also constitute 

 an important organ of excretion, especially in the Opisthobranchs 

 and Pulmonates. In the latter the dorsal wall of the pedal gland 

 is also excretory, and finally veritable accumulatory excretory 

 organs are often formed in the conjunctive tissue by plasmatic 

 cells known as the "cells of Leydig." True nephridia exist in 

 developmental stages in the form of "larval kidneys." 



