SECT. V THE NERVOUS SYSTEM 79 



to the fact that the eyes have thus travelled on 

 to the dorsal surface. Zaddach's diagram gives the 

 brain and oesophageal commissures a distinct curve 

 backwards, so that they come to lie along the 

 oesophagus, reaching as far back as the mid-gut. 

 The brain lies between the hepatic-diverticula. 



In discussing the appendages, we saw how the 

 antennae, which were originally metastomial, became 

 prostomial by the bending of the head. We now 

 see that the same change of position has taken place 

 in the case of the points of departure of their nerves 

 (at least of those of the second pair). By the sweep- 

 ing round of the cerebral portion of the oesophageal 

 commissures upwards and backwards, and by their 

 carrying the antennal nerves along with them, these 

 nerves have also acquired a prostomial position. 



This derivation of the central nervous system of 

 Apus from that of a bent Annelid throws new light 

 upon the fact, established by Claus and Dohrn, and 

 referred to by Balfour with evident surprise, that, in 

 the Nauplius, the nerves for the second antennae arise 

 from the infra-oesophageal ganglion. This is of 

 course what we should have expected, indeed from 

 our point of view it is necessary to account for the 

 fact that the nerves of botJi the pairs of antennas do 

 not arise from the infra-oesophageal ganglion. We 

 are obliged to assume that this primitive Annelidan 

 condition was passed through in embryonic and not 

 in larval life, i.e.^ in an early Annelidan, not in a 

 Crustacean stage. 



Of the rest of the nervous system little need be 



