THE CRAYFISH 135 



Like that of Apus and of Annelids it consists of a pre-oral 

 cerebral ganglion connected by a pair of pericesophageal con- 

 nectives with a ventral nerve chain. The points which should 

 be particularly noticed are the following : The cerebral ganglion 

 is a compact mass from which both the antennary as well as 

 the optic nerves are given off. Adopting the same reasoning 

 that has already been applied in the case of Apus, we must 



xi DC nr-VQ 



Fig. 31 



Central nervous system of the crayfish. cer, supra-oesophageal or cerebral 

 ganglion ; ce, oesophagus ; ggl, right green gland ; ^/, bladder of left green 

 gland. III-VII. Subcesophageal ganglion mass supplying nerves to the 

 mandibles, maxillae, and first two maxillipeds. VII. Ganglion supplying 

 the third pair of maxillipeds. IX-XIII. Ganglia supplying the remaining 

 thoracic segments. XIV-XIX. Ganglia supplying the abdominal segments. 



regard the cerebral ganglion of the crayfish as a syncerebrum, 

 into which the two antennary ganglia have been incorporated. 

 Thus the cerebral ganglion or " brain " of the decapod is a 

 step in advance of that of the phyllopod in respect of integration, 

 for in the latter only the first antennary ganglia are incorporated 

 with the brain, those of the second antennae retaining their 

 more primitive position at the sides of the mouth. A similar 

 tendency to integration of the thoracic ganglia is clearly marked 

 in the crayfish. The pericesophageal connectives are united 

 behind to a large ganglionic mass from which nerves are 

 given off to the mandibles, maxillae, and first and second 

 maxillipeds. Immediately behind this, and scarcely dis- 

 tinguishable from it, is a second ganglionic swelling supplying 

 the nerves of the third maxillipeds, but there is a distinct 

 ganglion pair for each of the remaining segments from the 

 ninth to the nineteenth inclusive. It is obvious that the first 

 five postcesophageal ganglia have been fused to form the anterior 

 thoracic ganglion, while the ganglion of the eighth segment is 



