54 MOLLUSC A. 



whereas the eyes on the tentacles in the same animal are 

 purely Molluscan in their structure. 



The Gasteropoda have no organs of smell, though it is 

 well known that they seem to seek and find food by this 

 means. Perhaps some part of the moist, exposed skin, 

 which is supplied with nerves, acts in this way, since it is 

 well known that, besides the nerves, moisture and air are 

 necessary to this function. Let the scholars hold their 

 breath and drink anything, they cannot taste it until they 

 draw in a current of air over the tongue, unless it is excep- 

 tionably bitter, or astringent. 



The common Garden Slug, Limax maximus, which 

 may be procured for a trifle from any horticulturist, is 

 also an air-breather (Fig. 29). 



Observe in this the saddle or fleshy cushion (;;/) on 

 the back, the orifice on the right side as in the snail, 

 and the similar way in which it opens to admit air (Fig. 

 29). This saddle is all that is left to represent the 

 mantle. Open it with a sharp-pointed knife or scissors, 

 and you will find a little homy disc inside (Fig. 52). 

 Hold this up against the light, and it will show the 

 usual concentric lines of growth. It is in fact a flat- 

 tened shell, one which has retained the character of 

 the young or embryonic shell plate, and which, instead 

 of being externally covered only by the epidermis, has 

 been enveloped during growth by a fold of the mantle 

 enclosing it completely in the adult.* 



Further dissection will show similar organs to those 

 described in the snail, but all of these are concentrated 

 in the crawling disc. 



* Above Fig. 52 is shown an imaginary section through a 

 slug at this point. 



