136 COMPARATIVE ANATOMY 



partially fused with the compound mass. It should further 

 be noted that whereas in Apus the ganglia and nerve cords 

 of the ventral chain are widely separated, and the space 

 between them is bridged over by relatively long transverse 

 commissures, in Astacus the elements are so much approxi- 

 mated in the middle line that the members of each pair of 

 ganglia are fused together, and the double nature of the 

 connectives is only apparent on close inspection. They 

 diverge, however, between the ganglia of the eleventh and 

 twelfth segments to admit of the passage of the sternal artery. 



The abdominal ganglia lie beneath the ventral or flexor 

 muscles of the abdomen, and are exposed when these muscles 

 are removed ; but the thoracic ganglia lie in an arcade formed 

 by the projection of a number of skeletal pieces into the 

 cavity of the body. These pieces must be removed in order 

 to see the thoracic nerve chain; though they appear to lie 

 inside the body they are really infoldings of the external 

 cuticle, and are shed along with the other hard parts during 

 ecdysis. 



In addition to the brain and ventral nerve cord, there is a 

 visceral or stomatogastric nerve system, consisting of a median 

 azygos nerve running back from the ventral surface of the 

 brain between the perioesophageal connectives to the roof of 

 the proventriculus. Here it is joined on either side by a pair 

 of nerves arising from a stomatogastric ganglion on the con- 

 nectives, and is continued backward as a median nerve lying 

 on the roof of the proventriculus : near the pylorus it branches 

 right and left to form the lateral nerves, each of which is 

 joined by an infero-lateral nerve coming from the stomato- 

 gastric ganglion, and a branch to the liver is given off from 

 their point of union. 



We may now consider the organs of special sense. The 

 olfactory hairs, or " Leydig's organs," on the first antennae 

 have already been mentioned, and the position of the orifice 

 of the auditory sac has been described. The auditory organ 

 itself is a small sac with thin chitinous walls. The posterior 

 end of the sac is pointed, and just above it the auditory nerve 

 a branch of the antennary divides into two branches and 

 spreads along the posterior and inferior walls of the sac. 

 Beneath the branches of the nerve the walls of the sac are 

 raised internally into a curved ridge whose sides are beset 



