THE GASTROPODA Til 
in Phyllirhoé (Fig. 161, 1). As a rule the kidney is a compact mass, 
without external projections, but it is divided into two lobes in 
Stenoglossa in general and also in some Taenioglossa, viz. Paludina 
and Cypraea. In a fairly large number of Nudibranchs (Dorido- 
morpha, Janus, etc.) the kidney is divided into ramifications which 
extend between the visceral organs of the greater part of the body 
(Fig. 79, XIII). In sundry Pectinibranchs—e.g. Littorina—there 
is a “nephric gland” which opens into the kidney, and consists 
of ciliated canals surrounded by conjunctive tissue. In addition to 
its excretory function the kidney may also serve for the conduction 
of the genital products. Thus in all Gastropoda with two kidneys, 
that is to say, in all the Aspidobranchia (Pleuwrotomaria, Trochus, 
Fig. 55, Fissurellidae, etc.) except the Neritacea, the gonad opens 
into the right kidney by a papilla situated near the external renal 
aperture. 
(2) The pericardial glands in the Aspidobranchs and Valvata 
are placed on the external walls of the auricles. In other forms 
they are localised on the internal wall of the pericardium, as in 
Littorina and Cyclostoma among the Pectinibranchs and in the 
Pleurobranchidae and Nudibranchia among the Opisthobranchs, or 
they are situated within the pericardium on the origin of the aorta, 
as in Aplysiidae. 
(3) Various excretory products may be accumulated in plasmatic 
cells (known as the “cells of Leydig”) in the conjunctive tissue of 
different parts of the body. This phenomenon is particularly 
common on the walls of arterial trunks, and may be seen in the 
caudal artery of Curinaria, and on the wall of the arterial trunks of 
certain Streptoneura and many terrestrial Pulmonates, in which 
caleareous concretions are found in the perivascular conjunctive 
tissue. The different forms of excretory apparatus and the special 
function of each can be revealed by the method of physiological 
injections. 
4. Nervous System.—With the exception of the endoparasitic 
Entoconchidae, all Gastropods possess a well-developed nervous 
system in which the same cerebral, pedal, pleural, visceral, and 
stomato-gastric nerve-centres, and the same connectives and com- 
missures, are to be found as in other Molluscs. But the special 
character of the Gastropod nervous system is the asymmetry of the 
visceral centres and of the nerves arising from them, an asymmetry 
resulting from that of the visceral organs themselves. The most 
primitive form of nervous system is characterised, as in the Poly- 
placophora, by the absence of concentration in the ganglia. The 
cerebral centres in the Rhipidoglossa are situated at the sides of 
the oesophagus and are united by a long commissure which is 
itself ganglionated (Fig. 94). The pedal centres in Aspidobranchs 
(Fig. 94, pe.c), Paludina, and some other taenioglossate Pectinibranchs 
