286 MOLLUSCA [CH. 



the surface of the foot are contractions of the vertical (dorso-ventral) 

 muscles connecting the foot with the body as each part of the 

 foot in succession is lifted from the substratum the transverse fibres 

 contract and by forcing forward the blood cause this part to lengthen 

 and move forwards. Then as the wave passes this part acquires a 

 new adhesion to the substratum, the longitudinal muscles then 

 contract and pull the rest of the foot after the extended part. 



The head of the snail bears two pairs of feelers, or tentacles, 

 which are hollow outgrowths of the body-wall (2, 3, Fig. 127) : these 

 when irritated are protected by being pulled outside in, and so 

 are brought into the interior of the body. The first or shorter pair 

 are supposed to be the chief seat of the sense of smell : the second 

 and longer pair have at their tip a small pair of black eyes. These 

 eyes are merely minute sacs, the walls of which are made of light- 

 perceiving cells, connected at their bases with a nerve which leads 

 to the brain ; in the cavity of the vesicle is a horny lens which 

 nearly fills it up. The eyes of nearly all the Mollusca are con- 

 structed on the same plan, but in'the Cuttle-fish not only is the 

 vesicle large and spacious and the lens proportionately smaller, but 

 there is in addition a series of folds of skin surrounding the place 

 where the eye comes to the surface, which constitute an outer 

 chamber, and outside this, eyelids, so that the whole organ acquires 

 a superficial similarity to the human eye, 



If we carefully pick away the shell of the animal and lay bare 

 the visceral hump, brushing away any mucus which may adhere to 

 the body, we shall see on the right side of the animal a round hole 

 (5, Fig. 127). A bristle passed through this reaches into a large 

 cavity separated from the outside by an exceedingly thin wall. 

 This space is nothing but the mantle-cavity, which, as explained 

 above, is the space comprised between the projecting mantle flap 

 and the rest of the body. The peculiarity about the snail is that 

 the mantle edge has become fused to the back of the neck so as to 

 shut the mantle-cavity off from the exterior, leaving only this little 

 hole of communication. The mantle-cavities of the marine allies of 

 the snail, such as the whelk and periwinkle, are not so completely 

 shut off, inasmuch as in them the mantle flap merely lies against 

 the neck but is not fused to it, and inside the mantle-cavity there 

 is a gill. This gill consists of a hollow axis bearing on one or both 

 sides a close set row of thin plates inside which the blood circulates 

 and receives oxygen from the water by diffusion. Fresh supplies of 

 water are drawn into the mantle-cavity by the action of myriads of 



