THE MOLLUSCA 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 Cyclostoma, 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.” 
