n. m 



$t. 



212 THE CRAYFISH. PHYLUM ARTHROPODA 



commissures pass backwards to join behind the oesophagus in the 

 subcesophagcal ganghon. This gives nerves to the Hmbs as far 

 as the second maxilhpeds, inclusive, and immediately behind 



it lies the first thoracic 

 "^///f ganglion, which supplies the 



/^ ^ third maxillipeds. In each of 



the remaining segments of 

 the thorax lies an indistinctly 

 double ganglion which sup- 

 plies by several nerves the 

 limbs and other organs of its 

 segment. These ganglia are 

 set at some distance apart and 

 are connected by double com- 

 missures, forming thus a ven- 

 tral cord. Between the fourth 

 and fifth ganglia the com- 

 missures part widely to allow 

 the sternal artery to pass 

 between them. In the ab- 

 domen the cord is continued 

 and consists of a ganglion in 

 each somite united to its 

 fellows by longitudinal com- 

 missures, which are really 

 double, but appear at first 

 sight to be single. The last 

 ganglion supplies the telson 

 as well as its own somite. 

 The commissures contain no 

 nerve cell bodies. The brain 

 is more complex than those 

 of annelids and exercises more 

 control over the rest of the ner- 



A semi-diagrammatic view of VOUS Systcm. Giant fibres run 



from cells in it along the whole 

 length of the cord and enable 

 it to bring about sudden move- 

 ments which involve distant 

 parts of the body, such as the 

 backward escape movement. 



Fig. 144. 



central nervous system of a crayfish. 



ab.i, ab.b, Thf first and sixth abdominal ganglia ; cer., 

 cerebral ganglion ; c.oes., circumoesophageal 

 commissure ; I.e., longitudinal commissures of 

 ventral cord ; n.ab.l., nerves to abdominal limbs ; 

 n.a.ti, nerve to antennule ; n.at.2, nerve to antenna'; 

 n.ch., nerve to cheliped ; n.w., nerves to limbs 

 adjoining the mouth ; o.n., optic nerve ; s.ces. 

 ''"bo ,1 ganglion; st.a., sternal artery; 



'*i| ■ l and sixth thoracic ganglia; v.n., 



nerve to proventriculus ; v.n'., nerve to hind-gut! 



