INVERTEBRATE PHOTORECEPTORS 655 



ceans with polarized light, had found no such analyzer function. And 

 apparently none need be sought. The obliquity of the ommatidial axes 

 at the surface of the convex compound eye serves adequately. Seemingly 

 this furnishes the information needed by the central nervous system of 

 xiphosurans, ants, and bees — at least — in their abilit}^ to align their 

 courses according to compass directions. Only in the fly Volucella has a 

 claim been made that individual receptor groups could detect the plane 

 of polarization (Menzer and Stockhammer, 1951) ; whether this too is 

 due to obliquity of ommatidial axes cannot be learned from the descrip- 

 tion given. 



Anatomical studies of arthropod compound eyes have been very numer- 

 ous, but certain publications contain large enough blocks of fundamental 

 information so that they have become the classics of the field : Grenacher 

 (1879), Notthaft (1881), Watase (1890), Exner (1891), Hesse (1901b), 

 Demoll (1910, 1917), Best (1911), Hanstroem (particularly 1926), and 

 Bernard (1937). For the most part, these retain a general flavor. Other 

 reports are limited more to special classes and orders within the phylum. 



Of the crustaceans, decapods and stomatopods have compound eyes 

 on stalks and make compensatory movements of the eyestalks when 

 the animal or its visual field is rotated. Amphipod compound eyes are 

 usually as unspectacular as those of Gammarus, but in one group (the 

 Hyperina) hypertrophy has produced enormous eyes occupying a major 

 part of the enlarged head (as in Hyperia and Phronima), and in another 

 [the ampeliscids (see della Valle, 1888; Svensson, 1934)] the eyes are 

 divided into separate parts, and some of these degenerate (as in Haploops) ; 

 in cavernicolous types degeneration has often proceeded as far as complete 

 eyelessness. The Mendelian genetics of pigment loss is known in one 

 species of Gammarus (Allen and Sexton, 1920). Most isopods have rela- 

 tively coarse compound eyes, like those of the terrestrial AnnadilUdiiim, 

 Oniscus, and Porcellio. But shallow and deep-water species show a range 

 of structure (Beddard, 1884), and abyssal forms may completely lack 

 eyes. Degeneration among cavernicolous isopods appears to be much 

 more variable within each species (Banta, 1921; Kosswig and Kosswig, 

 1936). In the females of Cymothoa, which are parasitic in the mouths of 

 fishes, degeneration is progressive during each life history (Eggert, 1927). 

 The sole cumacid studied (Diastylis) has a median fused compound eye 

 composed of eight ommatidia, four from each original eye. Some other 

 genera in this group have two eyes, a fused median structure, or lack 

 eyes entirely; nocturnality is common, and mud burrowing is a frequent 

 habit. Among barnacles, compound eyes form during the metanauplian 

 stage but, depending on the species, are either extruded to fall away or 

 absorbed at the time of metamorphosis into the adult (Fales, 1928). 



Among entomostracan crustaceans all gradations can be found between 

 genera with a distinct pair of compound eyes and genera in which the 



