236 TONGUES OF MOLLUSCA. PART m. 



than the Conchifera or bivalve class. Their mantle forms 

 a vaulted chamber over the head and neck, and envelopes 

 the foot or crawling-disk ; all these the animal can pro- 

 trude or draw in at pleasure. The head is of a globular 

 form, with two or four exceedingly sensitive tentacles, 

 arranged in pairs on each side of it, as in the garden 

 snail, which has four, two long and two short. These 

 tentacles, which the snail can push out and draw in at 

 pleasure, are hollow tubes, the walls of which are com- 

 posed of circular bands of muscle. The tentacles are 

 pushed out by the alternate contractions of these circular 

 bands, but they are drawn in again like the inverted 

 finger of a glove by muscular cords proceeding to the 

 internal extremity of the tentacle from the muscle that 

 withdraws the foot. The structure of the tentacles is 

 the same in all the crawling mollusks ; they are most 

 sensitive in the Helix or Snail family, but they are be- 

 lieved to be delicate organs of touch in all. 



The Gastropod mollusks never have more than two 

 eyes, either placed on the tips, or at the base of one pair 

 of tentacles ; in the snail they may be seen as black 

 points on the tips of the longest pair. In some of the 

 higher Gastropods they are of great beauty, and appear 

 to be perfectly adapted for distinct vision. Organs of 

 hearing were discovered by Dr. Siebold at the base of 

 one of the pairs of tentacles, consisting of vesicles con- 

 taining a liquid and calcareous otolites, which perform 

 remarkable oscillations due to the action of vibratile cilia. 

 In the Snail and Slug group the number of otolites varies 

 from eighty to one hundred. 



The mouth of a Gastropod is a proboscis, with fleshy 

 lips, generally armed with horny plates or spines on the 

 jaws. The Snail has a crescent- shaped cutting plate on its 

 jaw, and a soft bifid lip below, but the tongue is the most 

 remarkable microscopic object in this group of Mollusca. 

 In the terrestrial Gastropods, it is short and entirely 

 ontained within the nearly globular head. It is tubular 



