364 



PHYSIOLOGY OF THE INVERTEBRATA. 



with the base of a crystalline coue. Each cone passes 

 inwardly into a nerve-rod, and then thickens into a striated 

 spindle-shaped body. The inner extremities of the striated 

 spindles become narrow again, and then pass into the optic 

 nerve (Fig. 70). 



k 



Fig. 70. — TiiK Eye ok thk Dix apoua. 



A = ICye of Astacus. B = Eye of Honianis, a = cornea. /' = crystalline 

 cones. c = nerve rods. d = striated bodies. e — optic nerve, 

 y" = lenticular bodies, s; = fenestrated membrane. // = layer not present 

 in Astacus. k = pigment cells between cones. C = cornea of Dccapoda. 

 D — cornea of Iitsecta. 



There are certain species of crayfishes which are blind ; 

 among these may be mentioned Camharus setosns (Faxon), 

 which lives in the caves of south-western Missouri, and 

 Camharus pdlucidus, the well-known species from the Mam- 

 moth Cave. Mr. G. H. Parker* has recently examined the 

 question of degeneration of these organs. He states that 



* Bulletin of the Museum of Comparative Anatomy at Harvard College, 

 vol. 20 (1890). 



