THE COMPOUND EYE 



165 



short outer, a main intermediate, and a hollow inner segment. The 

 retinule consists of relatively few cells (4 in crabs) grouped around the 

 central rhabdome, the proximal extremity of which rests on a fenes- 

 trated membrane. There is evidence that the retinular cells are not all 

 of the same kind ; thus 3 different types have been described in the 

 Isopod. Ligia (Ruck and Jahn, 1954). In some species (the crayfish. 

 Astacus, and the shrimp, Crago) the nuclei of the retinular cells are 

 arranged in three zones, a configuration somewhat reminiscent of the 

 multi-layered retina of Vertebrates ; it is to be remembered, however, 

 that all are derived from the hypodermal cells 

 of the integument. 



The pigmentation is complicated, for each 

 ommatidium possesses at least two functionally 

 different pigments. Pigmentary cells (ikis 

 cells) containing melanin surround the distal 

 part of each ommatidium ; the proximal part is 

 similarly ensheathed or the retinular cells them- 

 selves also contain melanin ; while at the level 

 of the retinule is a clear reflecting pigment ^ 

 contained in separate cells ; this by reflection 

 prevents the entry of oblique rays. Although 

 the pigmentary cells do not move, the melanin 

 pigment within them shows marked migratory 

 changes (Welsh, 1930-41 ; Parker, 1932 ; 

 Bennitt, 1932) (Fig. 148). In bright light the 

 black pigment in the iris cells meets that in the 

 retinular cells so that the entire ommatidium 

 is encased in a sleeve of pigment ; in dim light 

 the pigment in the iris cells migrates distally to 

 lie between the cones, that in the retinular cells 

 migrates to a position proximal to the basement 

 miembrane, while the reflecting particles sur- 

 rounding the retinal elements, cleared of absorb- 

 ing pigment, act as a functional tapetum. We 

 have already seen that the migration of these 

 pigments often sho\\'s an autochthonous diurnal 

 rhythm - and that, in addition to this response 



to the direct action of light, they are under a complex hormonal and 

 nervous control (Kleinholz, 1936-38 ; Welsh. 1939-41 ; Brown, 1944 ; 

 and others).^ 



1 The chemical nature of the reflecting pigment varies. In the crayfish, Astacus, 

 the iris tapetum is of uric acid, in the lobster, Homnrus, uric acid is supplemented by 

 at least 3 other substances, none of which is guanine (Kleinholz and Henvvood, 1953 ; 

 Kleinholz, 1955). 



2 p. 19. * See further p. 554. 



Fig. 148. — The Ommati- 

 muM OF THE Cray- 

 fish, Astacus. 



On the left, in the light- 

 aflapted, and on the right, 

 the dark-adapted state. 



a, Cornea ; b, hypo- 

 dermal corneal cells ; c, 

 body of crystalline cone ; 

 d, inner segment of crys- 

 talline cone ; e, retinal 

 pigment cells ; f, rhab- 

 dome separating retinular 

 cells; g, tapetal cells; h, 

 basement membrane (mo- 

 dified from Bernhards). 



Ligia 



