INVERTEBRATE PHOTORECEPTORS 



633 



hyaline cells. These latter contribute a "vitreous body" separating the 

 photosensitive mechanism from the lens. Brunotte (1888) described 

 the anatomical details of these compound eyes in Branchiomma. Von 

 Buddenbrock (1934) examined the behavior of the worm without finding 

 anything spectacular — it became photokinetic when facing a black screen 



PELECYPOD (ARCA) 



WP^Sr^eJ- \-\n^ OF SECTI 



MANTLE 

 EPITHELIUM 



ANNELID (BRA NCHIOMMA } 



PIGMENT CELL 



OPTIC NERVE 



CRUSTACEAN (ASTACUS) 



SENSORY CELL 



GANGLIA 



CUTICLE 



OPTIC NERVE 

 FIBER 



SENSORY 

 /CELL 



OPTIC NERVE 

 FIBER 



Fig. 14-5. Grouping of visual units is found in annelids, moUusks, and arthropods, but 

 is most familiar in the last-mentioned phylum and is most diversified and specialized 

 there. In the annelid Branchiomma each unit includes several sensory cells, closely 

 analogous in arrangement to those of such crustaceans as Astacus; both of these group- 

 ings are hence compound eyes composed of ommatidia. In the moUusk Area, by 

 contrast, the units are unicellular eyespots, and the grouping is a compound eyespot. 

 Sections are shown at three different levels through an ommatidium of Astacus. 

 {Annelid after Hesse, 1899; pelecypod after Kuepfer, 1915; crustacean after Giesbrecht, 

 1921.) 



or facing away from a white screen but otherwise showed no reaction! 

 The camera-like alciopid eye, with its muscles allowing accommodation 

 by shifting the sensory tissue with respect to the dioptric apparatus, has 

 been described by Greeff (1875, 1877) and others, but again the function 

 is not known to attain the limits allowed by its organization. The 

 accommodation feature was investigated by Hess (1918) and Demoll 

 (1909a). A similar eye in Eupolyodo rites, according to Pflugfelder (1932), 

 is positioned in such a way as to be usable for binocular vision and depth 



