NERVOUS SYSTEMS OF INVERTEBRATES 15 



head, although there is no distinction between thorax and ab- 

 domen. The central nervous system consists of the usual nerve- 

 ring and double ventral cord, and well-developed ganglia are 

 present, between which the two halves of the cord commonly 

 remain distinct (fig. 1018). In Centipedes there is a certain amount 

 of fusion between the ganglia at the front end of the cord, the 

 region from which spring the nerves of the three pairs of jaws, 

 and also those of the poison-claws. 



MERVECORO 



Fig. 1017. Dissection of Peripatus from 

 the upper side, to show Central Nervous 

 System. I.A., Intestinal aperture. 



Fig. 1018. Dissection of a Centipede (Lithobius) 

 from above, enlarged 



In regard to Arachnids, it will be sufficient for our present 

 purpose to remark that the relation between the nervous system of 

 an elongated form, such as a Scorpion, with that of a shortened 

 form, such as a Spider or Mite, is much like that existing between 

 a Crayfish and a Crab (p. 14). For in a Scorpion many of the 

 pairs of ganglia of the ventral cord remain distinct, though there 

 is a good deal of fusion between those at its front end, while 

 in a Spider or Mite all the ganglia of the cord have consolidated 

 into a single nerve-mass. 



Among the Insects, again, we find the same principles exem- 



