SECT, vi THE SENSORY ORGANS U, 



We confine our attention at present entirely to the 

 paired eyes, reserving the unpaired " eye " for special 

 description later on. We may here say that, whether 

 this attempt to explain the origin of the Crustacean eye 

 as a visual organ from the Annelidan eye succeeds or 

 not, our theory will not be affected, for there are 

 points in the anatomy of the eye of Apus, such as the 

 musculature and the space between the eye and the 

 integument for water, which are easily enough ex- 

 plained on our theory, and which would, we think, be 

 very difficult to explain on any other theory. If then 

 we ourselves fail to trace the rise of the elements of 

 the Crustacean eye, another, better fitted for the task, 

 will no doubt be more successful. 



As already stated in the introductory chapter, our 

 original Annelid is supposed to have had (like the 

 Nereidae) two pairs of eyes on the prostomium, which 

 we have called simple eye-spots. Von Graber has 

 shown that these Chaetopodan eye-spots are by no 

 means simple structures, but are complicated visual 

 organs. This, however, does not make our task any 

 the more difficult, because \ve attribute the trans- 

 formation of the Annelidan into the Crustacean eye 

 chiefly to the thickening of the cuticle, which is one of 

 the Crustacean characteristics of the Nauplius before 

 the paired eyes are formed. The simple hypodermal 

 elements of the Annelidan eye have thus had to 

 develop, in each individual, not under Annelidan but 

 under Crustacean conditions, i.e., under a thickening 

 exoskeleton. 



When we come to ask what are the most character- 



