ASSOCIATED TYITH CONTRACTILE GLANDS. 373 



rapidly into a rift for shelter. They eat nothing but sand, 

 which they shovel into the gullet through the mouth, as do 

 some of the EcLinoderms ; of course they only digest the nutri- 

 tions organic particles which are mixed with the sea-sand. 

 Thus, in order to find suitable nourishment, they must often 

 be exposed to the gaze of the swift fish that leap rapidly 

 along the edge of the sea, as well as to that of other animals. 

 Flee they cannot ; a house into which to creep like many other 

 molluscs living equally exposed they have not ; they have 

 neither spines nor jaws with which to defend themselves ; and 

 the eyes on their hack, which are capable only of warning 

 them of the approach of danger, do not avail to provide them 

 with protection; in short, even with these dorsal eyes they 



FIG. 100.- PrHopfithalmns Kotlreateri, a fish which pursues OnchMinm a land mollusc 

 on the sea-shore. The large ventral nus serve for a forward leap. 



seem to be perfectly defenceless against their pursuers. Still, it 

 would certainly be very strange that such eyes should be deve- 

 loped here, and only in this particular genus, without being 

 qualified to be of any real advantage to them, since they cer- 

 tainly cannot require eyes on their back useful, no doubt, for 

 looking up to heaven, but quite useless for looking down on 

 earth in order to find their food, which lies close before them 

 in the sand under their mouth. 



Hence, if these eyes were in fact to be of some service to the 



mollusc, it must have been also provided with some sort of 



nd in point of fact such weapons do exist in every 



species that has such eyes. The skin of the hick is very thickly 



set with minute glands, of which the contents are not perfectly 



