XI 



PHYLUM ARTHROPODA 



493 



immediately behind the mouth, and sends off visceral nerves which 

 join to form a ring round the gullet, swollen in front into a small 

 visceral ganglion (v. gn.). Passing backwards, the nerve-chain 

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

 3t pair of abdominal feet (Fig. 390). 



The origin of the nerves given off from the central nervous 

 stem presents many points of interest. From the fourth ganglion 

 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 receive nerves, each from their own pair of 



FIG. 392. Shell-gland of Apus, diagrammatic, ac. cephalic artery ; b. bladder ; h. heart 

 m. second maxilla ; is. end-sac ; uc. urinary tube. (From Bernard.) 



ganglia, their serial homology with the more typical appendages 

 being thus confirmed. But the second maxillae receive their 

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

 ganglia : the ganglion belonging to their segment may be assumed 

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

 which springs from the oesophageal 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 jaws 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 prae-oral. The nerve of 

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

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



