GASTEROPODA OF THE UNITED STATES. 49 



several times longer and thicker than the inferior pair. 

 The}^ are conico-cjlindroid in shape, with the free extre- 

 mity or point dilated, or bulbous, in the outer side of 

 which the eyes are placed. The inferior pair are short, 

 conico-cylindroid, and slightly bulbous at the point. 



The integument is thick at the base of the tentaculoe, 

 but gradually becomes thinner as it approaches the free 

 extremity, where it is delicate and transparent. The 

 color is the same on the general investment of the body, 

 except at the free extremity of the tentacul?e, where, 

 from its transparency, the structure beneath shows 

 through, and appears whitish. On the superior tenta- 

 cul^ it is rougher than upon the inferior, from the poly- 

 gonal 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 tentacula; takes place by means 

 of the contraction of the retractor muscle, which forms 

 within the tentaculae a cvlindrical 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 tentacula first disappears from view 

 and, in protrusion, is last to appear. The latter move- 

 ment takes place through the relaxation of the retractor 

 muscle, and the gradual contraction of the circular, mus- 

 cular fibres forming the basis of the integument, com- 

 mencing at the base of the tentacular and proceeding 

 towards the free extremity, by which course of movement 

 the latter is pushed out. 



7 



