INVERTEBRATE PHOTORECEPTORS 629 



by many anatomists. Yerkes (1902, 1903, 1906), Morse (1906, 1907), 

 and Murbach (1907, 1909) studied the light reactions and structure of 

 Gonionemus. Murbach (1907) concluded that the light reactions showed 

 the marginal sense organs to be only part of the photosensitive mecha- 

 nism. Light appeared necessary for upward swimming but acted as a 

 general photokinetic stimulus, with gravity providing orientation and 

 direction. 



CTENOPHORA 



Photosensitivity has been demonstrated only indirectly in comb jellies. 

 Heyman and Moore (1925) and Moore (1926) approached light sensi- 

 tivity of Beroe and Mnemiopsis, respectively, in terms of inhibition of 

 luminescence and found clear indication of dark adaptation. 



PLATYHELMINTHES 



The conspicuous ocelli of Planaria and other free-living turbellarians 

 have been described in detail. In all the organ consists of one or more 

 receptor cells surrounded by a cup-shaped group of pigment cells. In 

 Prorhynchus the ocellus is at its simplest, with a single pigment cell 

 cupped around a single photosensitive cell, the latter composed of an 

 inner rhabdom with the nerve-fiber endings, a middle elhpsoidal lens, 

 and an outer transparent "myoid" section containing the nucleus. 

 Kepner and Foshee (1917) noted considerable change in the gross form 

 of this ocellus as it became dark- or light-adapted. Hesse (1897) investi- 

 gated the angular coverage of the ocellus in Planaria. Other workers 

 experimented with removal of the eyes unilaterally or bilaterally and 

 found that photokinesis continued without eyespots but that direction- 

 ality was lost. Unilateral "blinding" resulted in loss of directional 

 responses from that side but no circus movements. Dermal photosensi- 

 tivity (structural basis unknown) seemed responsible for spectral sensi- 

 tivity. Beuther (1926) claimed a difference in response to red and to 

 yellow both over Hering's colored papers and to filtered light beams; 

 Erhardt's repetition (1932) and extension of this work explained all reac- 

 tions on the basis of brightness discrimination. Werner (1926) noted 

 that a considerable part of the sensitivity to ultraviolet was as a dorsal 

 dermal photosensitivity. He believed that responses were to energy in 

 ultraviolet wave lengths directly, not to secondary fluorescence in the 

 visible spectrum. Merker and Gilbert (1932a,b) found strong responses 

 to ultraviolet (366-313 m.^) but much less response to fluorescence; the 

 planarians exhibited orientation, photokinesis along the resultant of two 

 opposed beams of ultraviolet, and response to ultraviolet alone when 

 between opposed beams, one of which was ultraviolet and the other a 

 fluorescence-eliciting wave length. Response to ultraviolet was pro- 

 nounced also in contrast with an incandescent-lamp beam. Merker and 



