PHOTOSENSITIVITY IN INVERTEBRATES 633 



PELECYPOD (ARCA) 



ANNELID (BRA NCHIOMMA ) 



PIGMENT CELL- 

 SENSORY CELL- 



MANTLE 

 EPITHELIUM 



OPTIC NERVE 



CRUSTACEAN ( ASTACUS) 



GANGLIA 



CUTICLE 



OPTIC NERVE 

 FIBER 



SENSORY 

 /CELL 



OPTIC NERVE 

 FIBER 



FIG. 6. Quasiradial divergence of photosensory units is characteristic of both compound eyespots 

 and compound eyes. The former are exemplified by the pelecypod moUusk Area (upper right^ and 

 the polychaete annelid Branchiomma QeJC), shown in lengthwise and transverse section and in detail. 

 Each ommatidium of the compound eye Qower right, detaiO consists of a cuticular lens, additional 

 dioptric components on the ommatidial axis, a cluster of receptor cells whose nerve fibers penetrate 

 the basal membrane and an investing sheath of pigment cells. .At the extreme right are sections cut 

 through such an ommatidium at lesels as indicated. Commonly the optic nerve fibers pass to a 

 series of ganglia close to the eye Qower right, longitudinal section of crustacean eye and stalk). [Braji- 

 chiornn after Hesse, Area after Kijpfer, Astaciis after Giesbrecht; from Milne & Milne (193).] 



pods which molt after acquiring compound eyes re- 

 tain most of the dioptric mechanism, shedding only 

 the corneal lens or a part of it. 



There is reason to question that the con\entional 

 classification of arthropod ominatidia has a sound 

 phylogenetic basis. The term 'exoconc' is applied to 

 those of crustaceans, trilobites, and beetles of the 

 families Dermestidae, Elateridae and Lampyridae, in 

 which the dioptric parts consist of a molted corneal 

 lens and a nonmolted inward extension of corneal 

 secretion. Elsewhere special cone cells ('Semper's 

 cells') lie between the corneal lens and the receptor 

 cells, and provide dioptric function. In 'acone' om- 

 matidia the cone cells become transparent and refract 



light; they occupy all of the space and are character- 

 istic of the insect orders Dermaptera, Heteroptera, 

 some Odonata, some Coleoptera and .some nemato- 

 cerous Diptera. In 'eucone' ommatidia the cone cells 

 secrete a solid 'crystalline cone' within themselves, 

 usually in such a wa\' that the cone-cell nuclei remain 

 distal to the cone; .sometimes an anuclear portion of 

 the cone cells lies proximal of the cone; this type is 

 characteristic of the insect orders Thysanura, CoUem- 

 bola, Orthoptera, Homoptera, Neuroptera, Trichop- 

 tera, Lepidoptera, Hymenoptera, some members of 

 Odonata, inost of the Coleoptera and some neinato- 

 cerous Diptera. The brachycerous Diptera are unique 

 in ha\ing 'pseudocone' ommatidia in which the cone 



