74 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



its transparency, the structure beneath shows through, and appears 

 whitish. On the eye-peduncles it is rougher than upon the tentacula, 

 from the polygonal folds being deeper. It is but loosely attached to the 

 parts within, except at the free extremity, where it becomes firmly united. 



The retraction of the eye-peduncles takes place by means of the con- 

 traction of the retractor muscle, which forms within them a cylindrical 

 tube, and is inserted into the integument at their free extremity, so 

 that when they are retracting the integument becomes inverted, and 

 the point of the eye-peduncle first disappears from view and, in protru- 

 sion, is last to appear. The latter movement takes place through the 

 relaxation of the retractor muscle, and the gradual contraction of the 

 circular muscular fibres forming the basis of the integument, commen- 

 cing at the base of the eye-peduncle and proceeding towards the free 

 extremity, by which course of movement the latter is pushed out. 



Within the tube of the retractor muscle of the eye-peduncles, passes 

 to the free extremity of the latter the tentacular nerve. When the eye- 

 peduncles are retracted, the nerve becomes tortuous and spiral, but 

 when fully protruded it is nearly straight or merely undulated. 



Near the free extremity of the eye-peduncles, the nerve undergoes a 

 sudden constriction, and then dilates into a gangliform enlargement, 

 from the outer side of which proceeds a small division of the tentacular 

 nerve, as the optic nerve, to the eye. The gangliform enlargement is 

 composed, on the exterior, of the nerve-tubuli of the tentacular nerve, 

 and on the exterior, of a soft, white, finely granular matter, containing, 

 in the exterior layer, round, granular, nuclear bodies. Anteriorly, the 

 enlargement undergoes a constriction, and then dilates into the large 

 bulbous mass of the extremity of the eye-peduncles. This latter mass 

 is white, soft, and finely granular. Upon its exterior the nerve-tubuli 

 of the exterior of the first gangliform enlargement diverge, and divide 

 into a number of large branches, which laterally subdivide into numer- 

 ous smaller branches, and thus enclose the granular mass. 



The tentacula present the same nervous structure, except that there 

 is no well-marked constriction between the tentacular nerve and the 

 first gangliform enlargement, nor between the two enlargements, nor is 

 there any optic nerve. 



The space between the tentacular nerve and the retractor muscle is 

 filled with a filamentous tissue, containing round, granular, nucleolated 

 nuclei, and large, round or elliptical, transparent cells, with nuclei sim- 

 ilar to those which lie free in the tissue. 



