SECT; xi DEVELOPMENT 157 



the original Annclidan prostomium has been discussed. 

 \Yc also saw how it, together with the paired eyes, 

 wandered on to the dorsal surface. In support of this 

 migration of the eyes, we call attention to Figs. 36 

 and 37, which show the eyes paired and unpaired 

 far more anteriorly placed in the Nauplius than they 

 arc later in the adult, i.e. midway between tJie ventral 

 position in tJie bent Crustacean- Annelid and tJic dorsal 

 Crustacean position. 



In Limulus, as already mentioned, the ocelli travel 

 during embryonic life from the ventral to the dorsal 

 surface. The homology of the unpaired eye of Apus 

 with the two ocelli of Limulus assumed here and on 

 p. 1 08 will be further discussed in Part II. 



It is important to note that this sensory organ is 

 present in all Nauplii, and persists throughout life in 

 all Entomostraca, but degenerates in the Malaco- 

 straca. In the more highly developed larvae of these 

 latter, traces of it arc also generally, if not universally, 

 to be found, e.g. in the Phyllosoma larva of Palinurus, 

 the Erichthus larva of Squilla, and in some, if not all, 

 Zoaea larvae. Owing to this almost universal presence 

 of the unpaired eye among the Crustacea it has been 

 assumed that it was present in the original Crustacean. 

 This assumption falls in with our theory that it was 

 first developed in the Crustacean-Annelid. 



In addition to what was said on p. 109 as to the 

 function of this organ, we may say that its form as a 

 hollow vesicle full of pigment cells seems at first 

 sight to suggest an auditory organ, but we share 

 the growing conviction among zoologists that many 



