402 



GASTEROPODA. 



the interior of the body when they are not in actual employ- 

 ment. 



The only senses that we can expect to meet with in animals 

 deprived of either an external or internal skeleton, are those of 

 taste, smell, vision, and touch ; any auditory apparatus being of 

 course deficient. 



The sense of taste, judging from the structure of their tongue, 

 must be extremely obtuse ; and, although these creatures are evi- 

 dently possessed of smell, it is not easy to point out where their 

 olfactory apparatus is placed. The eyes, however, are now found 

 to present a perfection of structure correspondent with the enlarged 

 brain, and occupy a singular position, being situated at the ex- 

 tremities of the two superior tentacula appended to the head ; 

 while the inferior pair, adapted, as it would seem, more exclusively 



Jty.184. 



to the perception of tactile 

 impressions, are deprived of 

 visual organs. Both the up- 

 per and lower tentacula are 

 retractile, and can be com- 

 pletely inverted so as to 

 be withdrawn into the in- 

 terior of the body. To 

 effect the inversion by which 

 this end is attained, the plan 

 represented in the accompa- 

 nying figure is had recourse 

 to. Each tentacle is a hol- 

 low flexible cylinder, the 

 walls of which are muscular, 

 and composed of circular fibres. When partially retracted, as in 

 the tentacle marked c in the figure (fig- 184), the extremity of the 

 organ is drawn inwards, and two cylinders are thus formed, one 

 within the other : if the outer cylinder is elongated, as in pro- 

 truding the tentacle, it is at the expense of the inner one ; and, on 

 the contrary, the inner cylinder, when the organ is retracted, is 

 lengthened as the other becomes shorter. To evert the tentacle 

 the contraction of the circular muscles that form its walls is suffi- 

 cient, as they can gradually unroll the whole by squeezing out, 

 as it were, the inner portion ; but to effect its inversion a spe- 

 cial retractor muscle is required, which is represented in the ten- 

 tacle indicated in the figure by the letter b. This muscle (g) 



