PTEROPODA AND GASTROPODA. 551 



than usually minute or wanting, these ganglions are always larger 

 than in the Acephala, and more decidedly superior in position : they 

 supply the acoustic vesicles in many Gastropods. When separate, 

 they are united by a thicker communicating chord, and are larger in 

 proportion to the nerves given off from them. With the cephalic 

 ganglions, likewise, we find connected the labial and pharyngeal 

 ganglions. The anterior of the aggregated ganglions, which form 

 the sub-cesophageal mass in most Gastropods, are in immediate con- 

 nection with the acoustic vesicles in PleurobranchcBa and Paludina, 

 as in Clio and some other Pteropods. The functions of the other 

 ganglions of the body seem to be limited to the aatomatic reception 

 and reflection of stimuli. 



Soft, lubricated, and irritable as is the skin of the naked 

 Mollusks, there are not wanting reasons for supposing it to be 

 possessed of a very low degree of true sensibility. Baron Ferussac, 

 for example, states that ha has seen the terrestrial Gastropods, or 

 slugs, allow their skins to be eaten by others, and, in spite of large 

 wounds thus produced, show no sign of pain. 



The vascular inferior surface of the foot may, perhaps, take cog- 

 nisance of the character of the surface over which it glides ; but the 

 special organs of the tactile sense are the tentacula or horns which 

 project from the lateral and upper parts of the head. These are 

 wanting in a few Gastropods, hence called Akera : they are some- 

 times two, and never exceed four, in number in the present class. 

 In the snails and slugs they can be retracted by an act of inversion. 

 The mechanism by which this is effected wiU be understood by re- 

 ferring to Jig. 201., which exhibits the tentacles in different states of 

 protrusion. Each tentacle (b, c, d,) is here seen to be a hollow tube, 

 the walls of which are composed of circular bands of muscle, and 

 capable of being inverted like the finger of a glove. From the 

 common retractor muscles of the foot four long muscular slips are 

 detached, one (^) for each horn ; these run in company with the 

 nerve {/) to each tentacle, passing within its tube, when protruded, 

 quite to the extremity. The contraction of this muscle dragging the 

 apex of the organ inwards, as seen at c, inverts it, whilst its pro- 

 trusion is effected by the alternate contractions of the circular bands 

 of muscle of which the walls of each tentacle are composed. There 

 is, however, another peculiarity rendered necessary by this singular 

 mechanism, by which the nerves supplying the sense of touch may 

 be enabled to accommodate themselves to such sudden and extensive 

 changes of position ; for this purpose the nerves supplying these 

 organs are of great length, reaching with facility to the end of the 

 tubes when protruded, and in their retracted state the nerves are 



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