PHYLUM ARTHROPODA 



285 



anterior to the oesophagus. Each green 

 gland consists of a glandular portion, 

 green in color (40), a thin-walled dilata- 

 tion, the bladder (41), and a duct open- 

 ing to the exterior through a pore at the 

 top of the papilla on the basal segment 

 of the antenna (42). 



Nervous System. — The morphology 

 of the nervous system of the crayfish is 

 in many respects similar to that of the 

 earthworm. The central nervous system 

 includes a dorsal ganglionic mass, the 

 brain (Fig. 202, 25), in the head, and 

 two circumcesophageal connectives (26) 

 passing to the ventral nerve-cord (27), 

 which lies near the median ventral sur- 

 face of the body. The brain sends 

 nerves to the eyes, antennules, and an- 

 tennae. Each segment posterior to VII 

 possesses a ganglionic mass, which sends 

 nerves to the surrounding tissues. The 

 large subcesophageal ganglion in segment 

 VII consists of the ganglia of segments 

 III- VII fused together. It sends nerves 

 to the mandibles, maxillae, and first and 

 second maxillipeds. Visceral nerves arise 

 from the brain and extend posteriorly to 

 the viscera. 



Sense-organs. — Eyes. — The eyes of 

 the crayfish (Fig. 202, 28) are situated at 

 the end of movable stalks, one on either 

 side of the head. Each eye is covered 

 by a modified portion of the chitinous 

 cuticle called the cornea. The cornea is 

 divided into rectangular areas known as 



Fig. 203. — Longitudinal 

 sections of two ommatidia 

 of the crayfish. A, pigment 

 arranged as influenced by 

 light. B, pigment arranged 

 as influenced by darkness. 

 /, cornea; 2, nucleus of 

 corneagen cells; 3, nucleus 

 of vitrella ; 4, nucleus of 

 pigment cell; 5, crystalline 

 cone ; 6, tapetum cell ; 

 7, rhabdom; S, retinal cell; 

 p, basement membrane ; 

 70, retinal nerve fiber. 

 (From Sedgwick's Zoology, 

 after Parker.) 



