260 



MANUAL OF ELEMENTARY ZOOLOGY 



th. 1 



A transverse commissure immediately behind the oesophagus joins 

 the two circumoesophageal commissures. It contains fibres which 

 take this roundabout course between the portions of the brain which 



supply the antennse, thus 

 indicating that these limbs 

 belong to the same series 

 as those behind the 

 mouth. This is probably 

 also true of the anten- 

 nules, and the fact that 

 the antennules and an- 

 tennas are innervated 

 from the supracesopha- 

 n. geal ganglia must be con- 

 nected with the position 

 of the mouth, which, as 

 a result of cephalisation 

 s. oes. t° a hi g h degree (p. 233), 

 is farther back than in 

 the earthworm, where it 

 lies in front of the first 

 segment. The alimentary 

 canal is supplied by two 

 visceral nerves. The first 

 has a threefold origin, 

 being formed by the junc- 

 tion of a nerve from the 

 cerebral ganglion with 

 two nerves which arise 



Fig. 165. — A semi-dia- 

 grammatic view of 

 central nervous sys- 

 tem of a crayfish. 



ab.i, ab.6, The first and sixth 

 abdominal ganglia ; cer., 

 cerebral ganglion; c.as., 

 circumoesophageal com- 

 missure ; /.c, longi- 

 tudinal commissures of 

 ventral cord ; n.ab.l., 

 nerves to abdominal 

 limbs; n.at.i, nerve to 

 antennule ; tt.at.2, nerve 

 to antenna : n.ch., nerve 

 to cheliped ; w.w/., nerves 

 to limbs adjoining the 

 mouth ; o.n., optic nerve ; 

 J.rffj.jSubcesophageal 

 ganglion ; st.a., sternal 

 artery; th. i, 1/1.6, first and 

 sixth thoracic ganglia ; 

 v.n. , nerve to proven- 

 triculus; v.n'., nerve to 

 hind-gut. 



71. ah. I. 



