INVERTEBRATE PHOTORECEPTORS 641 



light. Smith (1906) found pigment migration in the retina of Planorbis 

 but uncovered no clue as to its causation. In the nudibranch Chromo- 

 doris the small eyes situated beneath the skin were believed an impor- 

 tant part of the receptive mechanism for general photic response, but 

 the gill crown responded to shading independently, apparently as a result 

 of sensory structures in the branchial collar. Even a single genus may 

 show great variation. Thus the species of Onchidium (a pulmonate 

 gastropod) examined by Crozier and Arey (1919; Arey and Crozier, 1921) 

 reacted on the basis of dermal sensitivity, with a lack of mantle eyes 

 and no photic mechanism in the tentacles. In other species of this same 

 genus and in related onchidiids the mantle is set either with short tuber- 

 cles bearing eyes or with eyes embedded in the general tissue, these eyes 

 having an inverted retina, a huge lens, and a mechanism allowing muscu- 

 lar accommodation. 



Pelagic heteropods such as Pterotrachea have even more peculiar visual 

 organs at the base of the tentacles (Fig. 14-7). A relatively enormous 

 spherical lens is supported a short distance inside a hemispherical cornea. 

 Small groups of visual cells lie at and near the bottom of an elongated 

 cone extending inward from the lens; some of these groups are on pro- 

 jecting ridges, and the term "ladder retina" has been applied to the 

 mechanism. Hess and Gerwerzhagen (1914) stimulated the eye electri- 

 cally and found muscular movements that shifted the lens with respect 

 to the retinas, presumably as a mechanism allowing accommodation. 

 Whether the minute retinas on the various ladder steps allow use of the 

 eyes as sights is not known. 



Cephalopoda. Cephalopod eyes have been studied extensively because 

 of similarity in form to that of the vertebrate eye. Krohn (1842) and 

 Langer (1850) pointed out that good muscles are present, allowing accom- 

 modation; Beer (1897) investigated this aspect quantitatively and noted 

 that no change in lens curvature occurred but that the lens itself was 

 shifted with respect to the retina. In most cephalopods the eyes are 

 disproportionately large; in Sepiola, for example, a specimen weighing 

 3.5 g had eyes each weighing 0.9 g. At rest the focus is correct only for 

 near objects, and accommodation is thus negative. Von Pflugk (1910) 

 confirmed these conclusions in Octopus, finding the animal myopic by 

 6-10 diopters. Hensen (1865, 1866) made comparative studies among 

 the cephalopods and considered the pinhole eye of Nautilus (Fig. 14-8) 

 to be degenerate in its lack of cornea, lens, pupil control, and internal 

 hyaline media. Embryological development, however, corresponds to 

 the initial steps in the formation of more complex cephalopod eyes. 



The cephalopod retina is not inverted, as is the vertebrate counter- 

 part, and the origin of the whole eye is quite different, but it is equipped 

 with muscles for shifting the eyeball. Heine (1907) and Alexandrowicz 

 (1927) studied accommodation possibilities by stimulating the eye elec- 



