PHOTOSENSITIVITY IN INVERTEBRATES 



629 



PLANAR I A 



DORSAL 



EPITHELIUM 



MEDIAL 



( IN CROSS SECTION ) 



FIG. 4. The dorsal ocelli of the turbellarian flatvvorm Planaiia consist of opaque pii^ment cups 

 open laterally, concealing the distal ends of photoreceptor cells. Shadowing of the photoreceptors 

 by the pigment cups differs in horizontal illumination according to the orientation of the worm 

 Qeft). Each receptor cell (right) is most sensitive to radiations passing through it parallel to the 

 long axis of the portion within the pigment cup. [.After Hesse; from Milne & Milne (193).] 



among the many eye-bearing members of the phyla 

 Annelida, Mollusca and Arthropoda that it is tempt- 

 ing to arrange them in parallel .series (i 17, 193, 237). 

 A phylogenetic basis for this series would be valuable 

 (fig. 5), Ijut no correlation has been found between 

 form of ocelli and other structural features or with 

 the normal habitats occupied. Hence it seems prob- 

 able that the variation has no Ijroader implications, 

 and the parallels in embryonic de\elopment are 

 fortuitous. 



The several paired ocelli on the prosiomium of the 

 polychaete Nereis are of a single structural type with 

 a cuticular lens over a cupped retina (162, 199). 

 The most anterior pair mav mediate negative re- 

 sponses to light and the others positive responses 

 (105); an asymmetry of the retina in the anterior 

 pair appears to adapt them to forward and lateral 

 vision, whereas the other ocelli are directed more 

 vertically upward (199). Brand (24) reported that 

 the behavior characteristic of unilaterally blinded 

 Nereis is shown even when any single ocellus is left 

 intact on the operated side. 



In the Atlantic palolo worm, the polychaete Eunice, 

 each segment bears a single mid-ventral ocellus, but 

 its function has not been found (in) since general 



photosensiti\ity appears to account for responses 

 observed. 



The ocelli of leeches appear to be the chief special- 

 ized sensory organs and in the first fi\e body .segments 

 occupy the positions corresponding to lateral-line 

 organs in more posterior regions. Each ocellus is al- 

 most cylindrical with its longitudinal axis at right 

 angles to the body surface; its nerve fibers connect 

 on the medial surface. Whether they are phylogeneti- 

 cally related to tactile elements (293) or can legiti- 

 mately be arranged in an evolutionary series (266) 

 has not been proved. Their function may be related 

 more to body pigment distribution (247) than to 

 kinetic responses. As Parker pointed out (205), mere 

 possession of photoreceptors does not imply that an 

 animal can see. 



The abundant small ocelli of amphineuran mol- 

 lusks provide a comparable puzzle. An adult Cure- 

 pliium may have as many as 8500 of these structures, 

 perhaps 3000 in the most anterior plate of the shell 

 (198, 214, 215). Heath (92) traced their formation 

 and concluded that they must be functional even in 

 the adult. Crozier (40) could find only a general 

 photosensitivity, however, in Chiton. It was most 

 pronounced in the scaly girdle, where ocelli are lack- 



