THE GASTROPODA m 



in Phyllirlioe (Fig. 161, I). 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 (Pleurotomaria, 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 Valvaia, 

 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 Cydostoma 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 Carinaria, and on the wall of the arterial trunks of 

 certain Streptoneura -and many terrestrial Pulmonates, in which 

 calcareous 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 



