SMELL 



On: 



Tent.N. 



Fig. 1038. Tip of Optic Tentacle of 

 Garden - Snail, in diagrammatic longitu- 

 dinal section, enlarged. The tentacular 

 nerve (Tent. N.} gives off an optic nerve 



olfactory patch (Olf.} of cells on the tip of 



the tentacle. 



There is naturally a tendency for olfactory organs to be 

 developed at the front end of the body, 

 where they can be most usefully em- 

 ployed, and they are commonly to be 

 found on the feelers of Arthropods. In 

 the Crayfish, for example, the small first 

 feelers (antennules) bear groups of flat- 

 tened bristles which undoubtedly have 

 to do with smell, and similar structures 

 are present on the antennae of Millipedes 

 and Insects (fig. 1037). 



\ o J / / (Op.N.} to the eye, and then expands into 



Land-Snails and Slugs, among the a g an s lion ( Gn -^ which sends fibres to an 

 Molluscs, are known to be endowed 

 with a keen sense of smell. 

 In the common Garden- Snail 

 {Helix aspersa) some of the 

 epidermic cells at the tips of 

 the long eye-bearing tentacles 

 are believed to minister to this 

 function (fig. 1038), though 

 experiments have been made 

 which appear to show that 

 olfactory cells are elsewhere 

 present. 



Among aquatic Molluscs 

 what is known as a water- 

 testing organ (osphradium) is 

 usually present in the neigh- 

 bourhood of the breathing 

 organs (fig. 1039). This is 

 generally considered to be of 

 olfactory nature. 



In Vertebrates the sense- 

 cells related to smell form part 

 of the lining of the cavities of 



, , , Fig. 1039. Diagram of a Comb-gilled Snail, seen from 



tile nOSe, and Since these are above. The roof of mantle-cavity and overlying shell sup- 



developed as pits in the ex- ^ d '"' . 



x i, Mouth; 2, brain ganglion; 20, nerve-cord connecting 



temal SUrfaCe, SUCh Cells mUSt side ganglion (above) with foot ganglion (below); 3, one of 



., - .. 1 . the three ganglia on the twisted nerve-loop; 4, gill; 4^, os- 



neCeSSarily DC OI eCtOCiermiC phradium; 5, opening of intestine; 6, heart in pericardium; 



character. When the sense of ^^^- s ^; n P^ ,,si P ho n: .,*. 



to 



