PHOTOSENSITIVITY IN INVERTEBRATES 



637 



size of each increase at each moh (284), rapidly in 

 early ages and more slowly later on. The same is 

 true in crustaceans studied (14, 16, 149) and most 

 insects (16, 173). The stick insect Dixippiis is unusual 

 in adding no new ommatidia, although the total in- 

 crease in dimensions of each is 126 per cent and the 

 eye area doubles from hatching to maturity. 



Development of the compound eye appears to de- 

 pend upon normality of the supraesophageal gan- 

 glion. Damage to this ganglion usually leads to failure 

 of the eye to differentiate. In Drosophila, however, the 

 various genetic mutants with degenerate eyes arise 

 through factors acting on the eye itself and not in- 

 directly through the nervous system (225). Degenera- 

 tion of compound eyes in cavernicolous arthropods 

 and deep-sea crustaceans is common and apparently 

 follows a similar genetic course influencing the eye 

 itself (83). Beddard (13) believed a relationship could 

 be seen between depth and degree of degeneration of 

 the compound eye, but so many instances of hyper- 

 trophy of these organs in deep-sea crustaceans have 

 been described that the generalization is unsafe. 



Regeneration following injury to the compound 

 eyes seems possible in decapod crustaceans, although 

 the regenerated part is not an eye but an antennalike 

 organ. Trilobites alone are known to have regenerated 

 ommatidia (138), this being recognized in terms of 

 independence in the direction of the facet pattern in 

 areas set off by scar tissue. 



CAMERA-STYLE EYES IN MOLLUSKS. The remarkable 

 convergence in anatomical organization between the 

 large eyes of some cephalopod moUusks and those of 

 vertebrate animals have led to frequent comment. 

 Hensen (102) investigated the embryonic steps lead- 

 ing to the cephalopod types of eye. In all the organ 

 arises as an invaginated vesicle. That of Nautilus is 

 unique in proceeding no farther and hence remaining 

 as a pinhole-camera eye (fig. 7, righl^. 



In all other cephalopods the vesicle closes and 

 sinks below the body surface. The douijle layer of 

 tissue where the pinhole closed produces a pair of 

 planoconvex lenses in contact with one another, as 

 the sole structure focusing an image in these marine 

 organisms. Distal to the lens an encircling ridge arises 

 forming the muscular iris diaphragm (fig. 7, center^. 

 The whole eye sinks further below the surface at the 

 bottom of a fresh invagination the rim of which 

 closes over either partialh' or completely in forming 

 a transparent cornea. A number of genera retain an 

 open pore between the anterior chamber and the 

 outside world, and sea water washes the front of the 



lens. In some genera an additional encircling ridge 

 forms around the eye, producing an approach to eye- 

 lids. 



Deep-sea cephalopods often have eyes which are 

 amazingly hypertrophied, sometimes supported on 

 swi\eling turrets (33). In these a binocular field seems 

 probable, whereas in most surface and mid-water 

 cephalopods the visual fields are completely .separate. 

 The apparent absence of blind cephalopods must be 

 related to the number of kinds which bear lumines- 

 cent organs in the depths. 



Most cephalopods have a slit pupil which closes 

 into a slightly hooked horizontal line. It is under 

 direct control from the central nervous system and 

 changes the degree of opening more in relation to 

 emotional conditions than it does refle.xly in relation 

 to light intensity (259). Muscles provide for accom- 

 modation of the lens (151, 164) and demonstrate their 

 action when the outer surface of the eye is stimulated 

 electrically (2, 101). In Octopus, at least, the resting 

 eye is myopic by 6 to 10 diopters, and accommoda- 

 tion is both positive for objects at clo.se range and 

 negative for distance (274). 



Unlike the vertebrate eye, the cephalopod organ 

 has a direct retina. Its optic nerve fibers may emerge 

 from the eyeball as multiple bundles which fuse into 

 a common optic nerve. Around them are the four 

 oculomotor muscles which shift the eye in a wide 

 range of movements, including rotational ones (121, 



25O. 



Electroretinograms from cephalopod eyes (211, 

 212) has'e been as helpful as beha\ior in indicating 

 the role of vision in these animals. In all cephalopods 

 the nervous system is .so highly organized, with \ isual 

 cues related elaborately to tactile ones and perhaps 

 taste as well, that simple responses are rarely elicited. 

 Captise animals are seemingly aff"ected strongly by 

 their confinement, but will develop conditioned re- 

 sponses under skilful handling. 



Camera-style eyes of quite different form are found 

 in some other mollusks. a) In the sand-eating pul- 

 monate gastropods Onchidium, Oncis and Peronina, the 

 dorsal surface of the body bears short wartlike projec- 

 tions each with a single eye or with from two to ses'cn 

 of them in an irregular cluster. Each eye is about 0.2 

 mm in diameter and has a two-part refractive body be- 

 tween the rather flattened cornea and the inverted 

 retina. The more distal refractive body alters in shape 

 when a muscular collar surrounding it contracts. 

 Presumably this is an accommodation mechanism. 

 Natural history observations on a Bermudan On- 

 chidium posses.sing eyes of this type suggest nothing 



