PHOTOSENSITIVITY IN INVERTEBRATES 



639 



GASTROPOD 

 (PTEROTRACHEA i 



CRUSTACEAN 

 (COPILIA ) 



VENTRAL VIEW 



SECRETION ACCOMMODATION 

 MUSCLE 



-OPTIC NERVE 



FIG. 8. Camera-Style eyes are found in several phyla of invertebrate animals, but the mechanism 

 of accommodation varies considerably. In the polychaete annelid Atciopa it includes both a hy- 

 draulic system from a secretory gland, shifting the lens distally, and a muscle operating in the re- 

 verse direction (Jeft). In the gastropod mollusk Plerolrachea, a muscle pro\ides the basis for fine 

 focusing on a series of receptor clusters known collectively as a ladder retina' (center). In the copepod 

 crustacean Copilia, a small group of receptor cells at the focal point of the biconvex lens is shifted 

 both toward the lens and swung laterally by muscular contraction. In both the mollusk and the 

 crustacean the eyes apparently are useful only as sights but, like the annelid eyes with their extrinsic 

 musculature, may have a binocular field in advance of the body. [Alciopa after DcmoU; Plerolrachea 

 after Hesse; Copilia dorsal view after Giesbrecht, detail after Grenacher; from Milne & Milne (193).] 



CAMERA-STYLE EYES IN ARTHROPODS. Still Icss Can bc 



guessed as to the function of strangely simplified 

 camera-style eyes in the planktonic copepod crusta- 

 ceans known as corycacids. Copilia carries two of 

 them facing forward, widely separated in the body. 

 Sapphirina has a pair close together. In Corycaeus their 

 lenses are fused on the mid-line. Yet in all, the large 

 lens in the body surface (fig. 8, right) appears to focus 

 light on a little cluster of three or four receptor cells 

 surrounded by a pigment sheath. A long slender 

 muscle lengthwise at the side of the eye can shift the 

 receptor cluster with reference to the lens in a way 

 which may provide for both some accominodation 

 and soiTie sighting, perhaps in binocular vision. 

 Nothing is known of the habits which would suggest 

 a use for a visual mechanism of this kind. 



PHENOMENA RELATED TO STIMULUS INTENSITY 



Changes in the sensitivity and in the discriminatory 

 capacity of multicellular eyes are often based in part 

 upon other features in addition to photochemical 

 changes and such obvious adjustments as those of iris 

 diaphragms. 



Pigment Migration within the Eye 



A redistribution of pigment, either by active exten- 

 sion and contraction of pigment cells or by shifting of 

 pigment granules within the protoplasm of stationary 

 cells, follows changes in intensity of illumination on a 

 variety of eyes: in the ocelli of the gastropod Planorbis 

 (4, 241); in the stemmata of the lepidopteran cater- 



