MOLLUSCA. 133 



mass, formed by the coalescence of several pairs of ganglia. This 

 ventral mass is divisible into postero-dorsal pleuro-visceral ganglia, 

 and antero-ventral pedal ganglia, respectively connected with the 

 cerebral ganglia by a posterior cerebro-pleural connective and an 

 anterior cerebro-pedal connective, on each side. 



The head is innervated by branches from the cerebral ganglia, 

 which give off five pairs of nerves. (1) The tentacular nerves, which 

 supply the optic tentacles. Each runs within the corresponding 

 tentacle, gives off an optic nerve to the eye, and ends in a tentacular 

 ganglion, from which branches run to the skin covering the end 

 of the tentacle. (2) Two pairs of labial nerves, one of which 

 gives off to the short tentacle a tentacular nerve ending in a 

 tentacular ganglion. These nerves supply the lips and neigh- 

 bouring parts. (3) An auditory nerve passes down on each side 

 between the two connectives to the otocyst. (4) A buccal nerve 

 comes off from the front of each cerebral ganglion, and runs 

 forwards to a buccal ganglion placed in the angle where the 

 gullet joins the buccal mass. The buccal ganglia innervate 

 the pharynx, gullet, and salivary glands. They are connected 

 together by a commissure ventral to the gullet. An unpaired 

 nerve runs to the penis from the right cerebral ganglion. The 

 body-walls and viscera are supplied by the pleuro-visceral ganglia, 

 while the nerves of the foot come off from the pedal ganglia. 



The nervous elements are as usual ganglion-cells and nerve-fibres 

 the former are confined to the outer parts of the ganglia, and 

 usually possess only one process. 



10. Sense Organs (1) Tactile Organs. The sense of touch 

 is possessed by the surface of the body generally, but is specially 

 localized in the head, tentacles, and sides of the foot. The tactile 

 cells are narrow cylinders with tapering external ends formed by 

 an aggregation of hair-like processes. 



(2) Olfactory Organs. The snail possesses a keen sense of 

 smell which enables it to detect the presence of various kinds 

 of food and to avoid certain strongly odorous liquids, such as 

 turpentine. This discriminative power is lost if the tentacles 

 are removed, and there is a patch of modified epithelium at the 

 tip of each of them, apparently of olfactory nature and containing 

 numerous flask-shaped cells closely related to the corresponding 

 tentacular ganglion. An olfactory function has also been ascribed 

 to the supra-pedal gland, a tubular organ, lined with columnar 



