GASTEROPODA. 



ders, one contained within the other, as shown in the annexed figure 

 (Jig. 191), the upper edges (z, z) of the two cylinders being con- 

 tinuous in such a manner, that, by drawing Fig. 191. 

 out the inner cylinder (i, 6), it becomes 

 elongated at the expense of the other, and, 

 on pushing it in again, it becomes shorter, 

 while the outer cylinder (k) is lengthened 

 by adding to its upper margin. 



The reader must now imagine a multitude 

 of longitudinal muscles (d, d), all very much 

 divided at both their extremities, and attach- 

 ed by one end to the parietes of the body, 

 whilst by the opposite they are fixed to the 

 interior of the inner cylinder of the proboscis 

 (b) along its entire length, and as far as its 

 extremity. It is evident that the action of 

 these muscles will retract this cylinder, and 

 consequently the entire proboscis, into the 

 body. 



When thus retracted, a great part of the inner surface of the 

 internal cylinder (b) will necessarily become a portion of the ex- 

 ternal surface of the outer cylinder (k) ; and the contrary when the 

 proboscis is protruded. It is in consequence of this that the in- 

 sertions of the muscles (d, d) vary in position. 



The protrusion of this proboscis is effected by the action of the 

 intrinsic circular muscles that form its walls. 



When the proboscis is extended, the retractor muscles (d, d), if 

 they do not act all together, serve to bend it in any direction, 

 thus becoming the antagonists to each other. 



In the internal cylinder are contained the tongue, with all its 

 apparatus (e, e) ; the salivary ducts (/), and the greater part of 

 the esophagus (g) : but the principal use of the proboscis is to 

 apply the end of the tongue to the surface of bodies that the Buc- 

 cinum wishes to erode and suck. The tongue itself (e) is a car- 

 tilaginous membrane armed with hooked and very sharp spines. 

 It is sustained by two long cartilages, the extremities of which 

 form two lips (c), that can be separated or approximated ; or the 

 cartilages can be made to move upon each other by the mass of 

 muscles in which they are imbedded. When these cartilages 

 move, the spines that cover the tongue are alternately depressed 

 and elevated ; and by a repetition of similar movements, aided per- 



