SECT, vi THE SENSORY ORGANS 95 



describe a possible development of the Annelidan 

 eye-spots into simple Arthropodan eyes, that being all 

 that we here need, in our endeavour to show that 

 Apus is a very primitive Crustacean, and at the same 

 time but a slightly modified Annelid. What we have 

 here written is a preliminary suggestion as to the 

 probable rise of the Crustacean eye. We hope in 

 another place, and in another connection, to discuss 

 it more fully, dealing especially with the sensory 

 elements and their physiological significance. 



Some further morphological details relating to the 

 paired eyes of Apus fortunately admit of more satis- 

 factory deduction from the Annelida than does the 

 fine structure of the eyes themselves. 



As to their position, we have two remarks to make: 

 i. We find them on the dorsal frontal surface, 

 whereas in the original Annelid they were on the 

 prostomium. It has already been assumed that, on 

 the fixation of the bent attitude of the five anterior 

 Annelidan segments, they gradually wandered round 

 on to the dorsal surface. There is no great difficulty 

 in this assumption, especially as we have seen, in our 

 investigation of the central nervous system, that the 

 position of the brain and the divided cesophageal 

 commissures indicate that such a wandering of the 



to us to find that he had also selected the compound eye of Limulus as 

 the nearest type of the primitive Crustacean eye. We do not see what 

 is gained by his assumption of integumental pits. It is not easy to see 

 how the various stages in the development of these pits could have 

 functioned as visual organs. 



