294 



MOLLUSCA 



[CH. 



there is a patch of thickened skin, called an osphradium (Gr. 

 ovfoaivofjMi, to smell), provided with numerous sense-cells, which 

 enables the animal to test the water which enters its mantle-cavity. 

 Of course no such organ exists in the snail. The muscles of the 

 radula are supplied by nerves from a special pair of small ganglia 

 placed on the buccal mass the buccal ganglia connected with 

 the supra-oesophageal ganglia. 



We thus see that the 

 nervous system of the 

 snail consists of a pair of 

 supra-oesophageal gang- 

 lia connected by commis- 

 sures with (a) a pair of 

 pedal ganglia supplying 

 the muscles of the foot 

 with nerves, (b) an ex- 

 tremely short visceral 

 loop, the ganglia on which 

 are so closely placed as 

 to become practically con- 

 fluent with each other, 

 whence nerves go to all 

 parts of the body, and 

 (e) a small pair of buccal 

 ganglia supplying the 

 buccal mass. The ner- 

 vous systems of all Mol- 

 lusca are built on this 

 plan : in the bivalves, 

 however, where there is 

 no radula, not only are 

 the buccal ganglia absent, 

 but the pleural and cere- 

 bral are fused with one 

 another, and, as the vis- 

 ceral loop is long, we find 



three widely separated pairs of ganglia, cerebro-pleural, pedal, and 

 visceral the last named often termed "parieto-splarichnic," in 

 different parts of the body. The Cuttle-fish have a closely massed 

 nervous system like the snail, which is protected in a kind of 

 rudimentary skull, made of cartilage. 



PIG. 135. Nervous system, osphradium (ol- 

 factory organ) and gills of Haliotis. After 

 Lacaze-Duthiers. 



1. Cerebral ganglion. 2. Pedal ganglion. 

 3. Osphradial ganglion. 4. Pleural 



ganglion. 5. Abdominal ganglion. 



7. Nerves to mantle. 8. Gills. 9. Pedal 

 nerves. 



