382 JOURNAL, BOMB AY NATURAL HISTORY SOCIETY, Vol. XVII 1. 



impulses. Boughly the ganglia may be compared to the receivers 

 and batteries of a telegraphic apparatus, the cords, to the wires. 



fisJ 



Fig. 7.— Nbrvous system of Sandhopper. 



If we take one of the lower Crustacese such as a Sandhopper, we 

 find it has a "brain" or supra-ce&ophageal ganglion to supply the 

 organs of special sense in the pre-oral region, and in each segment 

 behind the mouth we find a double cord with double ganglia lying 

 side by side in each somite. 



In the higher Crustacese we find a pair of ganglia in each seg- 

 ment, but they have coalesced to form a median mass. The double 

 chain of connectives have approached one another in the middle 

 line, but a separation can still be seen in the cephalo-thoracic region. 



In the Lobster we find a brain in front of the gullet ; two com- 

 missural nerves pass back on each side of the gullet to join a large 

 ganglionic mass which we find is the united ganglia of the mandi- 

 bular, both maxillary, and the three maxillipedary somites and 

 supplies all six pair of jaws and foot-jaws with nerves. A double 

 nerve commissure runs backwards to unite five other ganglia one 

 for each of the posterior five somites of the thorax, and supply the 

 five pair of walking legs. 



In the abdomen we find six ganglia, one for each somite. The cord 

 joining them requires very careful examination to discover it is 

 really a pair of strands. 



