ANATOMY AND PHYSIOLOGY OF A SNAIL. 



37 



recoils from strong smelling liquids such as turpentine, which it 

 invariably does when one or both are present. In Limncsa stag- 

 nalis and Flafiorbis the infra-cesophageal ganglia are orange or 

 red-coloured, and very pretty indeed. The nervous system of 

 Limnaa stagnalis well dissected out is the prettiest dissection I 



Fig. lo. — a, Nervous system of Haliotis (diagrammatic after Sprengel) ; Cg, cir- 

 cum-oesophageal mass ; Pg, pedal ganglion ; Pig, pleural ganglion (commissural 

 ganglion) ; Ag, abdominal ganglia ; O and 6>' olfactory organs ; Pe, pedal cord ; 

 S and 6" lateral nerves ; Br, gills ; /', nervous system of Linincea, (after Lacaze- 

 Duthiers). 



have ever seen. The circum-oesophageal mass in the large 

 Tritons is almost cartilaginous to the touch. 



The Olfactory Organs. — The sense of smell may exist in the 

 supra-pedal gland, the dorsal tentacles, or in the lobate processes 

 around the mouth. An osphradium is to be found in LinincEa, 

 Planorbis^ and Physa consisting of ganglion cells in relation with 

 a depression in the mantle-chamber above and behind the respira- 



