192 



GENERAL ZOOLOGY 



of the visual cells until it strikes the rhabdomes, which are the 

 receptors of the eye. The nervous impulse set up by the light 

 stimulus then travels back through the visual cells to the central 

 nervous system. 



Compound Eye. — Invertebrates show a 

 large variety of eyes which seem to have 

 image-forming possibilities. Of these the 

 most striking is the compound eye of 

 arthropods. In the crayfish and other 

 crustaceans these eyes are spherical in 

 shape and are mounted on the ends of 

 movable stalks. In an insect such as the 



ft 



~+ 1*3 "£? fit & 



vis. r. 



Fig. 125. 



Fig. 125. — Axial section of the eye of Planaria gonocephala. vis. c, visual 

 cells; vis. r., visual rods or rhabdomes; nv. /., centripetal fibers of visual cells; 

 pg. c, pigment cells. (From Dahlgren and Kepner, Principles of Animal Histology, 

 copyright, The Macmillan Company, after R. Hesse. By permission.) 



Fig. 126. — Longitudinal section of a single ommatidium of a roach, Periplaneta 

 orientalis. b. m., basement membrane; cu., cuticle divided into corneal areas; 

 rt. c, retinal cells; rh., rhabdome or cell organ of light perception; I., lens; I.e., 

 lens cells; c. c, corneal cells; sup. c, supporting cells; nv. f., nerve fiber. (From 

 Dahlgren and Kepner, Principles of Animal Histology, copyright, The Macmillan 

 Company, after R. Hesse. By permission.) 



grasshopper the eye forms a prominent rounded elevation on 

 cither side of the head. The outer surface of this type of eye 

 consists of a transparent cornea divided into thousands of poly- 

 gonal facets. In the crayfish the facets are quadrilateral in 

 shape and in the grasshopper they are hexagonal. Each facet is 



