PHYLUM ARTHROPODA 



209 



an anterior visceral nerve which arises from 

 the ventral surface of the brain; it is joined 

 by a nerve from each circumesophageal con- 

 nective, and, passing back, it branches upon 

 the dorsal wall of the pyloric part of the 

 stomach, sending a lateral nerve on each 

 side to unite with an inferolateral nerve 

 from the stomatogastric ganglion. 



Sense organs 



Eyes. The compound eyes of the crayfish 

 are situated at the end of movable stalks 

 which extend out, one from under each side 

 of the rostrum (Fig. 114). The external 



convex surface of the eye is covered by a 

 modified portion of the transparent cuticle 

 called the cornea. This cornea is divided by 

 a large number of fine lines into 4-sided 

 areas termed facets. Each facet is but the ex- 

 ternal part of a long slender visual rod 

 known as an ommatidium. 



Sections (Fig. 117, A) show the com- 

 pound eye to be made up of similar om- 

 matidia lying side by side, but separated 

 from one another by a layer of dark pigment 

 cells. The average number of ommatidia in a 

 single eye is 2500. 



Two ommatidia are shown in Fig. 118. 



Cornea y , 

 Facet A/ 



OJI 



Lighf rays 



rnea 



Pigment 



Nerve fibers 

 Optic ganglia 



Connective tissue 

 Optic ganglion 

 Muscle 

 Optic nerve 



B 



Figure 117. Crayfish. A, entire eye in longitudinal section to show its general structure. 

 B, diagram of part of a compound eye showing in strong light how the light rays are absorbed 

 by the pigment surrounding the ommatidia; only those that pass through the center such as 

 A-A', B-^', etc., reach the nerve fibers. This results in a separate image from each ommatidium. 

 (A after Borradaile and Potts; B after Lubbock.) 



Beginning at the outer surface, each om- 

 matidium consists of the following parts: 

 (1) a cornea (lens); (2) two corneagen 

 cells which secrete the cornea; (3) a crys- 

 talline cone formed by four cone cells; (4) 

 two retinular cells surrounding the crystal- 

 line cone; (5) several retinular cells which 

 form a central rhabdom where they meet; 



and (6) a number of black basal pigment 

 cells around the base of the retinular cells. 

 Fibers from the optic nerve enter at the 

 base of the ommatidium and communicate 

 with the inner ends of the retinular cells. 



Yision. The eyes of the crayfish are sup- 

 posed to produce a mosaic or apposition 

 image; this is illustrated in Fig. 117, B, where 



