530 



OASTEKOPODA. 



Fig. 265. 



withdrawn into the interior of the body. To effect the inversion by 

 which this end is attained, the plan represented in the accompanying 

 figure is had recourse to. Each tentacle is a hollow 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. 265), 

 the extremity of the organ 

 is drawn inwards, and two 

 cylinders are thus formed, 

 one within the other: if 

 the outer cylinder is elon- 

 gated, as in protruding the 

 tentacle, it is at the ex- 

 pense 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 contrac- 

 tion of the circular muscles 

 that form its walls is suffi- 

 cient, as they can gradu- 

 ally unroll the whole by 

 squeezing out, as it were, 



the inner portion ; but to effect its inversion a special retractor muscle 

 is required. This muscle (#) arises from the general muscular mass 

 composing the foot and retractile apparatus provided for drawing the 

 snail into its shell : the long slip of muscular fibres so derived, accom- 

 panied by the optic nerve (f), traverses the interior of the cylin- 

 drical tentacle quite to its extremity, where it is attached, and thus, as 

 the reader will easily conceive, is quite competent to cause its inversion. 

 The lower feeler (d) is represented in the figure as partly retracted by 

 the action of its appropriate muscle (&) ; while the corresponding one (a), 

 being completely turned inside out, is fully withdrawn and securely 

 packed among the viscera. 



(1401.) One circumstance connected with the contrivance above de- 

 scribed cannot but excite attention ; and this is, the peculiar arrange- 

 ment of the tentacular nerves, whereby they are adapted to changes of 

 position so extensive : the optic nerve (/), for example, must not be 

 stretched even when the eye-bearing tentacula are protruded to the 

 uttermost ; and in order to provide for this, when the feelers are not 

 extended, the nerves become thrown into close folds (h), and lodged 

 within the cavity of the body. 



(1402.) From the above somewhat lengthened account of the ana- 

 tomy of the Snail, the reader will at least have been able to become 



Structure of the tentacles of the Snail. 



