PHYLUM ARTIIROPODA 



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 circumcssophageal 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. 

 i, cornea ; 2, nucleus of 

 corneagen cells; ?, nucleus 

 of vitrella ; 4, nucleus of 

 pigment cell; 5, crystalline 

 cone; 6, tapetum cell; 

 7, rhabdom; S, retinal cell; 

 0, basement membrane ; 

 10, retinal nerve fiber. 

 (From Sedgwick's Zoology, 

 after Parker.) 



