224 SNAILS AND SLUGS [CH. VII 



part to disappear within the shelter of the rest of the 

 tentacle. Limncea and the other aquatic pulmonate 

 forms have a pair of eyes placed at the base of the single 

 pair of solid contractile tentacles upon their inner side. 

 The complex structure of the eyes would appear to 

 indicate better visual power than has as yet been demon- 

 strated by any experiments. There is strong reason to 

 believe that the general surface of the integument is 

 capable of appreciating differences of light and shade; 

 and there is little if any evidence to prove that the eyes 

 themselves are possessed of power superior to this. 



It is rather by the sense of smell that terrestrial 

 gastropods gain most information of the world around 

 them. Moquin-Tandon 1 has recorded that one wet day 

 he saw two slugs (Limax maximus) crawling from different 

 points towards a decaying apple; he moved the fruit to 

 the right, both slugs at once turned towards it, to the left, 

 and again the slugs changed course making once more 

 straight for the apple. When the tempting feast was held 

 over them the tentacles and body were raised towards it. 

 Many other observers have made similar experiments with 

 various slugs and snails, all tending to show that the 

 olfactory sense is extremely delicate and accurate. The 

 precise position of the olfactory organs has given rise to 

 much dispute. It seems probable that the specialised 

 epithelia with which both upper and lower pairs of tentacles 

 are provided at their apices are olfactory in function, and 

 the results of Griffith's experiments 2 lend strong support 



1 Mollusques de France, i. 



2 Proc. E. Soc. Edinb. xix. and No. 124, 1886-7. 



