638 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



PECTEN 



LOLIGO 



NA U TIL US 



FIG. 7. Mollusk eyes present a variety of optical systems. In the scallop Pectai Qejt), a cellular 

 lens concentrates light on two levels of inverted retina. The distal layer mediates only the oflT-responsc 

 leading to sudden closure of the shell valves. The proximal layer responds to steady illumination. 

 A reflecting tapetum (shown as opaque blocks basal to the pro.ximal retina) increases sensitivity and 

 contrast discrimination at low intensities of light. In the cephalopod NaulUus (rig/iQ, the eye becomes 

 functional and matures as a pinhole-camera organ at an embryonic stage passed through in the 

 development of all other cephalopod eyes. In the squid Loligo (center'), extrinsic muscles orient the 

 whole eye; intrinsic muscles provide both positive and negative accommodation and adjust the 

 aperture of the slit pupil. [Peclen after Kupfer, Loligo and Nautilus after Hensen; firom Milne & 

 Milne (193).] 



which might not be due to general photosensitivity 



(5)- 



A) Clams of the genus Cardium bear small eyes on 

 short tentacles around the rim of the mantle. Each 

 eye has a cellular lens mass in which the refractive 

 index changes from distal to pro.ximal and may pro- 

 vide an image on the rather coarse inverted retina. 

 A cup-shaped extension of pigment cells surrounds 

 the lens material and narrows to a distal pupil. The 

 whole eye is invested in a muscular coat, the contrac- 

 tion of which alters the shape of the lens mass and 

 may serve in accommodation. Nothing is known of 

 the function of these eyes. 



c) In the pelagic heteropods (gastropods) Ptero- 

 trachea, Carinaria and Atalania, one small eye is borne 

 projecting from the body contours on each side, like 

 the port and starboard running lights on a ship. 

 These highly modified eyes are directed forward and 

 must have a binocular field in front of the animal, 

 although no evidence has been presented to show 

 that the animal makes use of them in sighting on 

 objects of importance to it. Each eye has a large 

 spherical lens at considerable distance from a 'ladder 

 retina' with little ridges of receptor cells (fig. 8, 



centei) and a muscle who.se contractions shift the lens 

 (109), perhaps as a fine adjustment for focus. 



c.\MER.'^ -STYLE EYES IN ."ANNELIDS. Fairly conventional 

 camera-style eyes arc found in the pelagic polychaetes 

 Alciopa and Eupolvodonles. In the former (fig. 8, left) 

 the two eyes at rest diverge widely, but contraction 

 of three extrinsic muscles to each of them pro\ides a 

 basis for convergence, binocular vision and perhaps 

 distance estimation (45). In Eupolyodotites the eyes 

 face forward at rest (210). In all members of the 

 family Alciopidae, the eye structure is comparable 

 (79, 80). The large retina has direct receptors, a 

 secreted mass of two consistencies separating the 

 retina from the lens, an accommodation muscle (45), 

 and a secretory cell (107) which responds well to 

 electrical stimulation. Secretory action increases the 

 volume of the distal mass behind the lens and pushes 

 the lens forward, accommodating the eye for nearer 

 vision. Muscular contraction should operate in the 

 reverse sense. Unfortunately, no natural history de- 

 tails are available to indicate how and when these 

 worms use their remarkable eves. 



