EDIBLE SNAIL. ill 



The position of the anal, renal, and generative apertures in front and on the 

 right side of the body is due to the twist of the dorsal aspect of the body or visceral 

 dome, which is characteristic of Gastropoda Anisopleura. The presence of but a 

 single kidney and a single generative duct is nearly equally characteristic. 



The pulmonary chamber must be regarded as formed by the fold of the 

 mantle which in branchiate Gastropoda constitutes the roof of the branchial cavity 

 lodging the gills or ctenidia. The latter are aborted, and the mantle-fold itself has 

 become vascular and respiratory. The fore-edge of the mantle-fold in the aquatic 

 Pulmonate Limnaeus is free, and has not undergone fusion or ' concrescence ' with 

 the dorsum, as it has in Helix and its allies. When Limnaeus inhabits the deep 

 waters of lakes it is said to admit water to the pulmonary chamber, instead of 

 coming to the surface at intervals for a fresh supply of air, as it does when it 

 inhabits shallow streams and ponds. An adaptation of the branchial fold of the 

 mantle to aerial respiration occurs also in certain streptoneurous Gastropoda Aniso- 

 pleura : namely, in the Pneumonochlamyda and in the genus Ampullaria, among 

 Azygobranchia. In the last named, which is amphibious, and is found in tropical 

 America, Africa, and the East Indies, the left side of the branchial cavity contains 

 the ctenidium, and is separated by a fold from the right side, the walls of which are 

 vascularised. The Pneumonochlamyda^ to which our English Cyclostoma elegans 

 belongs, have lost the ctenidium, and respiration is carried on solely by the walls of 

 the branchial cavity, as in the Pulmonata^ from which order the Azygobranchia 

 differ in such essential features as the twisted character of the visceral nerve-loop, 

 and the separateness of the sexes. The view, which is advocated by von Ihering, 

 that the pulmonary chamber in Pulmonata is derived from the renal organ, does 

 not appear to be tenable. 



The size and extent of the pulmonary chamber vary greatly in the Pulmonata. 



The pericardial cavity is a portion of the coelome closed off completely 

 from the remainder, but communicating, as in all Mollusca where it is present, 

 with the exterior through the nephridium, with which it is connected by a ciliated 

 nephridial tube. The aorta divides into an intestinal and a cephalic branch, the 

 latter passing through the infra-oesophageal collar. The system of capillaries 

 appears to be very complete in Helix pomatia, judging from the result of injections. 

 The vascular system of Zonites algirus has been accurately investigated by Nalepa. 

 The arteries and capillaries have proper walls, and are lined by an endothelium. 

 The capillaries communicate by short branches with a narrow meshwork of wide 

 blood-spaces the ' transition ' vessels, which open into the venous spaces or the 

 coelome by infundibular orifices, seen also in Arion ater (=rufus) by Jourdain. 

 The venous system is represented in part by the coelome, in part by vessel-like 

 spaces, the walls of which are formed of homogeneous connective tissue with 

 scattered nuclei, but which are not lined by an endothelium. The pulmonary 

 vein pulsates rhythmically in Zonites. There is in the same Pulmonate a nervous 

 network in the walls of the auricle, of the ventricle, and aorta, in the last two 

 instances derived from the genital nerve. Nerves are found also in the walls of the 

 larger vessels. Ganglion cells connected with these nervous networks appear to be 

 rare. They are found in the walls of the heart of certain marine Rhipidoglossa 

 (Fissurella, Haliotis^ Trochus), according to Haller. See Haller, M. J. ix. 1883, 

 p. 61 ; cf. Dogiel, A. M. A. xiv. 1877. 



