PHYLUM ARTHROPODA 



535 



diminishes in size, and comes to an end at about the level of the 

 last pair of abdominal feet (Fig. 424). 



The origin of the nerves given off from the central nervous 

 system presents many points of interest. From the fourth ganglion 

 of the ventral cord backwards each pair of appendages has its own 

 pair of ganglia, the metameric correspondence between the limbs 

 and the nervous system being complete. The mandibles and the 

 first maxillae also Teceive nerves, each from their own pair of 

 ganglia, their serial homology with the more typical appendages 

 being thus confirmed. But the second maxilla? receive their 

 nerves (mn. 2) from the connectives between the third and fourth 

 ganglia : the ganglion belonging to their segment may be assumed 



FIG. 426. Shell-gland of Apus, diagrammatic, ac. cephalic artery ; ft. bladder ; /*. heart 

 MI. second maxilla ; ts. end-sac ; uc. urinary tube. (From Bernard.) 



to have atrophied. The antenna is supplied by a nerve (ant. 2) 

 which springs from the cesophageal connective, but which can be 

 traced backwards to the first ganglion of the ventral chain : this 

 fact may be taken as an indication that the antennae are serially 

 homologous with the ja\vs and feet that they are, in fact, meta- 

 meric or post-oral appendages which have shifted forwards, one 

 on each side of the mouth, thus becoming pre-oral. The nerve of 

 the antennule (ant. 1) also springs from the cesophageal connec- 

 tive, but is traceable forwards to the brain, where it is connected 

 with a special group of nerve-cells. This has been explained by 

 supposing that the antennule is a post-oral appendage the ganglion 

 of which has moved forwards along the cesophageal connective 

 and fused with the brain a process which actually takes place 



