CH. VII] SNAILS AND SLUGS 223 



when the animal expands again by the contraction of the 

 muscular integument, of which the floor of the pulmonary 

 chamber is a part, fresh air is drawn in. Many fluviatile 

 Pulmonates admit water within the respiratory chamber, 

 especially when living at considerable depths. It is probable 

 that many of them are able to avail themselves of bubbles 

 of oxygen given off by the leaves of aquatic plants 1 . There 

 is no doubt that the skin is an important though subsidiary 

 respiratory organ ; this is especially the case among slugs 

 and aquatic species. In many, e.g. Limax flavus, there are 

 integumentary pigments with respiratory functions and 

 having a marked affinity for oxygen. 



Excretion. The nitrogenous waste substances are elim- 

 inated by the renal cells of the nephridium. Urates of am- 

 monium and calcium are found in them and in the cavity of 

 the nephridial sac. According to Cuenot 2 the uric concre- 

 tions are discharged along the ureter at regular intervals of 

 about a fortnight. It is probable that some excretory 

 functions are performed by the walls of the pericardium, 

 with whose cavity that of the nephridium communicates 

 by the ciliated reno-pericardial canal. 



Sense Organs. The sense of sight, or at any rate 

 sensitiveness to light and shade, is possessed by all 

 snails and slugs. In the terrestrial species eyes are 

 situated at the summit of the upper pair of tentacles and 

 can be withdrawn within the tentacle. The retraction 

 consists of a " turning-outside-in " of the tentacle, the 

 process beginning at the apex, so that the eye is the first 



1 Gain, Journ. of Conch, v. 



2 C. R. Acad. Sci. Paris, cxix. 1894. 



