EYE-SPOTS IN DIGENEA. 



125 



TABLE SHOWING PIGMENTED EYE-SPOTS IN DIGENEA. 



GENERAL CONSIDERATIONS. 



The occurrence of eye-spots in various stages of the life cycle 

 of .Digenea and in representative species of all suborders of the 

 Prostomata at one time or another in the life cycle show the 

 fundamental significance of the eye structure. The preserva- 

 tion 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. The great trend has been one of 

 degeneration. If the sporocyst or redia ever possessed eye-spots 

 they have lost all vestiges of them. The absence of eye-spots in 

 many cercaria-trematode generations shuts out any clue of pig- 

 mentation at a former period. In some cases absolutely no 

 evidence exists in the life cycle of a possible former pigmentation. 

 In spite of these facts, the widespread pigmentation throughout 

 the Digenea warrants the belief that pigmentation was general 

 throughout the Class at some former time. 



In most cases pigment eye-spots have been described because 

 they were conspicuous. It is highly probable that in many 

 species pigmentless eye-spots exist. The three cases on record 

 (Cercaria racemosa, C. gracillima and C. minor] were found only 

 by a most careful study of the worms under high powers of the 

 microscope. While they are undoubtedly functionless, a record 

 of their existence extends the knowledge of the extent of eye-spots 

 and of the types of degeneration that have occurred under con- 

 ditions of endoparasitism. 



