NERVOUS SYSTEMS OF INVERTEBRATES 13 
some authorities hold a different view as to the antennules, be- 
lieving that these have always been situated in front of the mouth, 
and are in reality outgrowths from the head-lobe. If so they are 
comparable to the sensitive palps on the head of a Nereis, and 
the brain of Apus is strictly equivalent to the brain of an Annelid. 
We shall assume here the truth of the first 
view, as the balance of evidence is in its favour. 
The body of Apus is made up of a com- 
paratively large number of segments, while in 
Crayfish and Crab, as in all members of the 
highly-organized group (Decapoda) to which 
they belong, there are relatively few, ze. 
twenty, so far as can be definitely made out. 
Five belong to the Head, eight to the Thorax, 
and seven to the Abdomen, each with a pair 
of ganglia and, except the last, provided with 
a pair of limbs. The nervous system of the 
Crayfish has undergone a certain amount of 
fusion and centralization (fig. 1015). The brain 
is larger and more complex than that of Apus, 
and it supplies not only the first but also the 
second feelers, the ganglia corresponding to 
which have shifted along the nerve-ring. Even 
greater fusion has taken place at the front end 
of the ventral cord, where there is a large 
ventral ganglion, which has resulted from the _ 
union of the last three pairs of head-ganglia ga or ee Raa 
(supplying the three pairs of jaws), and the _ G., Gullet (in cross sec- 
é < . . tion); Bx, brain; W.R., side 
three first pairs of thoracic ganglia (supplying of neering; 7.G., ventral 
the three pairs of foot-jaws). It is interesting Sipe es al 
to notice that the third thoracic ganglia are erst, sensi ot Sh 
caught as it were in the act of uniting with 7%? anterior and posterior 
those in front of them. The last five pairs of 
thoracic ganglia (supplying pincers and walking-legs) are clearly 
defined, although by reference to the figure it will be seen that 
the last two are beginning to unite, while just in front of this 
the doubleness of the cord is practically demonstrated by the 
fact that its two halves diverge, for the passage of an artery 
which runs vertically downwards from the heart to supply the 
ventral region of the body. The first five pairs of abdominal 
