INVERTEBRATE PHOTORECEPTORS G31 



various stages in the life history of digeuetic forms. Faust (1918) con- 

 cluded that these findings demonstrated the fundamental significance of 

 eyespots, that the "preservation of the pigmentation and of the 'lens' in 

 this or that group has been decidedly erratic and apparently unrelated 

 to the systematology of the group," that the major trend was toward 

 degeneration, and that pigmentless eyespots no doubt exist in many 

 species. Three cases of this were reported {Cercaria racemosa, C. gracil- 

 lima, and C. minor); although they are undoubtedly functionless, the 

 existence of the unpigmented structures "extends the knowledge of the 

 extent of eyespots and of the types of degeneration that have occurred 

 under conditions of endoparasitism." 



NEMERTINEA 



Hilton (1921a) reported that photoreceptors were usually present along 

 the sides of the head — sometimes a single pair, more often in one or more 

 groups on each side. In the simplest condition these are mere eyespots, 

 but more complex ocelli have a limiting membrane around a fluid sup- 

 ported by strands from nearby cells. This mechanism serves to focus 

 light on sensory cells of somewhat rodlike form, enveloped in pigment 

 except toward the fluid sphere and connected to the cephalic ganglion 

 by nerve fibers. Minkiewicz (1906a, b) indicated strongly negative light 

 responses in Lineus, one of this group of worms. 



ASCHELMINTHES 



Among rotifers some genera possess no organized photoreceptors, but 

 in most there are at least two eyespots that, in their simplest form, con- 

 sist of a refractive globule in a red pigment cup to which nerve fibers pass. 

 In some these supposedly visual organs rest directly on the cerebral gan- 

 glion as "cerebral" eyespots; in others they are "apical" in position, 

 situated on the rostrum; in a few the corona (wheel organ) bears lateral 

 eyespots, and these may be combined with true ocelli in the other loca- 

 tions. Viaud (1938a, b,c, 1940, 1943) has been able to demonstrate that 

 rotifers reacting to light do so both through a general dermal photosensi- 

 tivity present even in eyeless forms (as shown by speed of locomotion) 

 and, where eyespots or ocelli are present, under the directing efi"ect of 

 these specialized organs. The two photosensitive systems, moreover, 

 show unlike spectral-sensitivity features that indicate either the effective- 

 ness of the red pigment or an actual dissimilarity in the photosensitive 

 basis of response. 



Among the nematodes, responses to light have been reported by 

 Wuelker (1924) without comment as to the mechanism involved. Hertel 

 (1904) included nematodes in his study of the 280-mM ultraviolet line 

 from the magnesium spectrum and concluded that catalytic formation of 

 hydrogen peroxide explained any responses shown. 



