II.] THE CRAYFISH AND LOBSTER. 227 



J. The nervous system. 



1. Pin the animal down tergal surface uppermost and lay- 

 bare the whole body cavity. Remove the heart, 

 reproductive and renal organs and the digestive gland ; 

 sever the alimentary canal across its middle and turn 

 the two halves to one side; cut short the tendon of 

 the adductor mandibuli muscle, and remove the flexor 

 abdominis, together with the endophragmal system. 

 The whole nervous axis will thus be laid bare. 



a. The supracesophageal ganglia; two considerable 

 masses, confluent in the middle line, situated at 

 the base of the rostrum immediately behind the 

 eye-stalks. 



b. The circumcesophageal commissures passing back 

 from a. 



c. The longitudinal ventral commissures ; backward 

 continuations of b, extending to the base of the 

 telson. They are widely separated in the mid- 

 thoracic region, where they enclose the sternal 

 artery (seen in transverse section. Cf. Sect. I. 3 d.) ; 

 elsewhere they are closely applied. 



d. The abdominal ganglia ; six paired masses, borne 

 upon c in the abdominal region. They are equi- 

 distant, there being one pair for each somite. The 

 posterior pair are larger than the rest, and distribute 

 fibres to the last somite and the telson. Note, in 

 relation — 



o. The ganglionic nerves ; paired trunks arising from 

 the ganglia. 



y8. The inter-ganglionic nerves; arising from the 

 longitudinal commissures, alternately with a. 



15—2 



