INVERTEBEATA. 



317 



thorax, where the sternal artery is ; this artery has to get 



to the ventral side of the nerve-chain, before it divides into 



anterior and posterior branches, 



and it is enabled to do so by 



the nerve-commissures diverging 



slightly on either side of the 



middle line to which the artery 



keeps. 



The metamerism of the nerve- 

 chain is less perfect than that 

 of the appendages, owing to the 

 fusion together of what we may 

 suppose to have originally been 

 distinct pairs of ganglia. Thus 

 while there are nineteen pairs 

 of appendages, there are only 

 fourteen pairs of ganglia. Of 

 these, the cerebral send motor 

 fibres to the muscles of anten- 

 nules and antennae, and must 

 therefore include representatives 

 of the ganglia of the first two 

 segments, which have shifted 

 forwards in front of the mouth. 

 Behind the mouth is a sub- 

 cesophageal ganglion, from which 

 motor fibres pass to the muscles 

 of the mandibles, first and second 

 inaxillse, and first and second maxillipedes. Here then we 

 apparently have five pairs of ganglia fused together. 

 Behind this there is one ganglion -pair to each segment 

 six thoracic and six abdominal. In the thorax the ganglia 

 are protected by the endophragmal arches, but in the 

 abdomen they lie just ventral to the flexor muscles. 



We see here a phenomenon similar to what we have seen already 

 in Vertebrata the tendency for the anterior somites of a metameric 

 body to become integrated or united in a compound structure in 

 which the metamerism is traceable only with difficulty (compare the 

 head of a Craniate Vertebrate with that of Amphioxus). The study 

 of a few other arthropod types would make this point clearer to tho 



Fig. 159. NEKVOUB SYSTEM OF 

 ASTACUS. 



(After Huxley.) 



