SECT, vi THE SENSORY ORGANS TII 



with the rowing limbs, and long before the paired eyes 

 arc developed or needed. It is also worth noting that 

 the unpaired " eye " is especially characteristic of most 

 small free-living Crustacea such as the Ostracoda, 

 Cladocera, and non-parasitic Copepoda. 1 In some of 

 these animals the organ probably combines rudi- 

 mentary visual with directive sensory functions, the 

 visual function being secondarily acquired, as there 

 can be little doubt that it has entirely ceased in 

 Apus. 



This view of the function of the unpaired eye, by 

 explaining its early appearance in the larva, makes it 

 unnecessary to suppose that it is, as is generally 

 assumed, therefore phylogenetically older than the 

 paired eyes. On the other hand, its appearance in the 

 larva of all Crustacea rightly leads to the conclusion 

 that it was present in the original racial form of the class. 

 According to our theory, Apus being the ancestor of 

 the majority of the modern Crustacea, the unpaired 

 eye appeared for the first time as such in Apus. 



A further and more exact study of this interesting 

 organ in Apus, and a comparison of it with the homo- 

 logous organs in other Crustaceans or Crustacean 

 larvae, is very desirable. It is impossible here to 

 follow up the matter further, as it would lead us too 

 far from the main subject of the book, which is an 

 endeavour to show how every single organ of Apus 

 admits of more or less easy derivation from similar 

 or dissimilar organs of a carnivorous Annelid. 



1 Some of the differences between the unpaired eye of Calanella 

 described by Grenacher, and that of Apus will be referred to in XV, 



