638 RADIATION BIOLOGY 



by stimulating the periphery of the girdle. Dark adaptation at night, 

 with increased sensitivity, was reported. 



Pelecypoda. In the peleeypods a very considerable range of photo- 

 sensory structures is found. Some, like the scallop Pecten, possess con- 

 spicuous eyes. Others, such as the clams Venus and Mya, are often 

 referred to as eyeless. Nagel distinguished between "skioptic" forms 

 such as Vemis, Ostrea, and others sensitive only to shadowing; "pho- 

 toptic" forms such as Lima and Mya which are sensitive also to bright- 

 ness level; "photoskioptics" such as Pholas which respond to both 

 increase and decrease in intensity; and "ikonoptics" such as Pecten in 

 which the structure supposedly allows image formation. Sharp (1883) 

 reported specialized pigment cells between the bases of the tentacles of 

 the external edge of the siphon in Solen and suggested that the shadow 

 reaction was due to their operation. Dubois (1889a,b) investigated 

 reactions in a similar system for Pholas. Corresponding mechanisms 

 are known in Mya (Hecht, 1921a, b,c; Light, 1930; Roller and von 

 Studnitz, 1934) from both structural and physiological viewpoints. 



A rudimentary pigment cup, sometimes with a lens, has been described 

 in Mytilus. In some species of Lima the mantle eyes consist of a cup- 

 shaped columnar epithelium of alternating retinula cells and pigment 

 cells ; the latter secrete a vitreous layer enclosing the inner knobbed ends 

 of the retinulae, and the optic nerve fibers collect together outside the 

 cup (Kuepfer, 1915). In Potamides, by contrast (Pflugfelder, 1930), 

 there is a distinct lens, a vitreous body surrounded by a single layer of 

 retinulae, but the nerve fibers emerge on the illuminated side of the 

 retina, so that it is inverted in the vertebrate sense. In Cardium also, 

 where the mantle bears many eyes, each has a thin cornea, a large multi- 

 cellular lens ovoid in form with its long axis parallel to the optic axis, 

 and a columnar retina separated from the lens by the optic nerve fibers 

 passing toward the optic nerve. The retinal cells are inverted also in 

 Pecten (Fig. 14-8), but there the retina is unusual in being double, with 

 both levels including photoreceptors directed away from the lens. Hart- 

 line (1938) studied electrical impulses in the optic nerve of Pecten when 

 the eye was illuminated and found that the proximal receptor layer 

 responded to steady illumination and that the distal layer generated an 

 off-discharge forming the presumptive basis of the animal's strong shadow 

 reaction. 



The lens in Pecten is a much more nearly biconvex body, separated 

 from the retina by enough distance to allow image formation. Wenrich 

 (1916) investigated this and found that the smallest white card, move- 

 ment of which produced a response in the scallop, was a 15-mm square 

 at 35-cm distance, indicating an acuity of 2° 28'. An alternative and 

 more probable explanation of the observed fact is that, at the light inten- 

 sity used, the appearance or disappearance of the card in the visual field 



