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. 
Fig. 1017. Dissection of Peripatus from Fig. 1018.—Dissection of a Centipede (Lithobius) 
the upper side, to show Central Nervous from above, enlarged 
System. 1.a., Intestinal aperture. 
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- 
