ASSOCIATED "WITE CONTRACTILE GLANDS. 373 



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

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

 some of the Echinoderms ; 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 back, which are capable only of warning 

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

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





V 



Fig. 100.— Pen'opJithalmif.', Kodrcutprl, a fish which pursues Onchidium^a land moUiisc — 

 on the sea-shore. The large ventral fins 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 auy 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 

 ill 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 

 weapons, and in point of fact such weapons do exist in every 

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

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



