INVERTEBRATE PHOTORECEPTORS 



639 



furnished the minimum effective change in intensity. A smaller card at 

 higher intensity or a larger card at lower intensity would probably have 

 been equally effective. Detailed anatomical knowledge of the eyes of 

 various species of Pecten is based on a series of papers from that by 

 Grube (1840) to those of Kuepfer (1915, 1916). A similar mechanism is 

 present in Spondylus. 



A very different visual structure is found in the attached Area, particu- 

 larly A. noae, and in the free-swimming Pectunculus. Thus Patten 



PECTEN L0LI60 NAUTILUS 



Fig. 14-8. MoUusk eyes provide for varying degrees of image formation. In tho 

 scallop Pecten, the two levels of sensory cells differ in function; those nearer the lens 

 mediate only an off-response, whereas those farther from the lens respond to steady 

 illumination. A reflecting tapetum (shown as dark blocks basal to the tips of the 

 receptor cells) increases sensitivity to low-intensity stimulation. In the squid 

 Loligo, reflex adjustments of a pupil, accommodation, and gross movements of the 

 eye through contraction of extrinsic musculature all provide a close analogy to the 

 vertebrate eye. The pinhole camera eye of Nautilus becomes functional and definitive 

 at a stage which is passed through in the embryonic stages of other cephalopod eyes. 

 (Pecten after Kuepfer, 1915; Loligo and Nautilus after Hensen, 1865.) 



(1886) recorded A. noae 8.5 cm long with 235 separate eyespots pro 

 jecting from the mantle margin. Each eyespot is either single or com- 

 pounded of as many as 70 per compound eyespot (Fig. 14-5). Each 

 unit in a compound eyespot is isolated from its neighbors by pigment 

 cells extending well up between the externally convex corneal-lens sur- 

 faces. Optic nerve fibers form a brushlike sensory net in the base of the 

 receptor cells, so that each is responsible for a sector of surrounding 

 environment in a manner analogous to the compounded ommatidia of 

 crustaceans and insects. 



"Eyes" described by Vaillant (1865) in the protruding mantle of the 

 giant clam Tridaena were referred to by Brock (1888) as glandular rather 

 than visual, but Yonge (1936) upholds the Hght-concentrating function 

 of these protruding hyaline bodies. Instead of a sensory function, how- 

 ever, these curious structures diffuse light wdthin thelmantle and permit 

 more rapid growth and reproduction of the algae embedded in the mantle 



