THE EYES OF MOLLUSCS 151 



form of eye from which the more highly differentiated types of simple 

 ocelli, compound, aggregate, and inverted eyes have been evolved. 



When differentiation in any particular line has already taken place 

 and become established, it is probable that — provided the environment 

 remains the same — it will persist, and that the development for instance, 

 of faceted eyes in one class will permanently distinguish that class from 

 another in which inverted eyes have been evolved. The branch of the 

 phylogenetic tree or class of animal which has developed faceted eyes 

 cannot be the parent stock of the branch which has evolved inverted 

 eyes, although both branches spring from a common trunk which includes 

 animals having a simple form of light-perceiving organ, which is capable 



Fig. 1 10. — Sketch of a Whelk (Buccinum undatum) in motion. 



eye at base of tentacle. /. : foot. op. : operculum, p. : proboscis. 

 s. : respiratory siphon or tube by which water is admitted to the gills. 

 (After Nicholson.) 



oc tent, 

 tent. 



gen.ap. 



Fig. hi. — Helix nemoralis, Snail, showing Paired, Simple Eyes at the 

 Free Ends of the Tentacles. (From Parker and Has well.) 



an. : anus ; gen. ap. : genital aperture ; oc. tent. : ocular tentacle ; pulm. sac : 

 pulmonary sac ; tent. : tentacle. 



of evolving in either direction, or may persist in its original form, as 

 seems to have been the case in the limpets, in which the two bilateral 

 cephalic eyes of the fully developed living types have the form of a simple 

 open pit, without lens. 



It is noteworthy that the nautiloid group, which was abundant in the 

 Palaeozoic era, included during this period a great variety of forms — 

 the shell being straight in Orthoceras — curved in Phragmoceras ; in the 

 form of a flat spiral with the turns not in contact ; or in a close spiral, 

 as in Nautilus ; or in the form of a cork-screw helix. Nautilus is apparently 



