SECT, xii RELATION OF APUS TO CRUSTACEA 199 



being very nearly pure archicerebra. They consist 

 almost wholly of the ganglia for the eyes and ocelli in 

 Limulus, and of the eyes and unpaired sensory body 

 in Apus. In Limulus, according to Packard, the brain 

 is not complicated by the presence of the ganglia for 

 the antennae ; in Apus, however, according to Pelse- 

 neer, the ganglia for the first antennae have joined the 

 brain. In the great simplicity of the brain, these two 

 animals are, Lankester states, almost unique. 



The eyes in Limulus, in wandering forwards and 

 outwards, were unable to take the brain with them, 

 but are simply connected with the brain by long, 

 and not very important, nerve fibres. These nerve 

 fibres have the same relative position on the brain as 

 the stalks of the optic ganglia of Apus. Between the 

 two optic nerves, a pair of nerves is found running 

 to the pair of ocelli which lie anteriorly near the 

 middle line. \Yc have already referred to these 

 median ocelli of Limulus, as some support for our 

 argument that the median sensory body in Apus 

 arose out of an anterior pair of eye-spots on the 

 prostomium of the original Crustacean-Annelid. 

 The position of the points of departure of the 

 nerves to these ocelli agrees exactly with that of 

 the nerves to the unpaired " eye " of Apus. In 

 Limulus there are other nerves leaving the brain from 

 between the optic nerves besides those to the ocelli. 

 In Apus we found that the sensory body is com- 

 posed of four retina:, with four nerves running to the 

 brain. If we homologise the lateral retina; with the 

 ocelli, the nerves from the postero-dorsal and ventral 



