200 THE APOD I D^ PART II 



retinae might correspond with a pair of the other 

 nerves just mentioned which leave the brain near 

 those of the ocelli. The different shape and grouping 

 of the sensory cells of the postero-dorsal and ventral 

 retinae from those of the lateral retinae seem to 

 indicate that they must have been derived from some 

 other sensory organs. 



Owing to the backward prolongation of the mouth 

 and the oesophagus, and the arrangement of the 

 limbs round the former, the anterior nervous system 

 is very concentrated ; the nerves for the anterior 

 antennae and the five pairs of limbs branch out 

 radially from the thickened cesophageal commissures. 

 It is as if the cesophagus had forced its way back- 

 wards between the two longitudinal commissures of the 

 nerve cord, forcing apart the separate pairs of ganglia 

 of the first five pairs of limbs, the four transverse com- 

 missures of which arch over the slanting cesophagus. 



Between the nerves to the fifth pair of limbs and 

 those to the operculum is a pair of nerves to the 

 chilaria or under lip. If the homology of the 

 chilaria with the under lip of Apus is correct, these 

 nerves have been carried back with the under lip, 

 in the drawing back of the mouth. 



The ventral cord of Apus is more primitive than 

 that of Limulus, which, at its posterior end, is much 

 modified. This specialisation of the ventral cord of 

 Limulus is in correspondence with the great concen- 

 tration of its body as compared with that of Apus. 

 In Apus the posterior end was found in a rudi- 

 mentary and larval condition. 



