WORMS 



189 



In some Rhabdocoela {Stenostonuni) curious hemispherical bodies consisting 

 of refringent granules lying underneath a bowl -shaped mass have been credited 

 with a photosensitive function ; there is no good evidence, however, for this 

 assumption. 



Eyes are lacking in the cave-dwelling planarians (Kenkiidae) and in endo- 

 parasitic Rhabdocoela.^ 



TREMATODES may possess simple ocelli in the larval stage (as in 

 the liver-fluke. Fasciola hepatica), but the adults, leading an essentially 

 parasitic existence, rarely possess sense organs. If they are present 

 they are of the simjDlest type, usually consisting of a single cell with a 

 striated border invested by a cup of pigment (Hesse, 1897 ; Andre, 

 1910 ; Faust, 1918) ; a typical example is seen in the luiicellular eye 



Fasciola hepatica 



Figs. 171 and 172. — The Eyes of Xemertine Worms. 

 E 



Fig. 171. — Lineus ruber. 

 E. eyes (after Hyman) 



Fig. 172. — The head of Ampkiporus 

 angulatus. E, eyes (after Hyman). 



of Tristomum impiUosum. a marine Trematode jiarasitic on fishes 



(Fig. 87). 



CESTODES, in keeping with their endoparasitic life, are without sense 

 organs. - 



Among the nemertines, most of which are freely-living and marine 

 in habitat, rudimentary eyes of the same subepithelial type as occur in 

 flat-worms are general and occasionally are very numerous (Figs. 17 1-72). 

 They are always limited to the anterior end of the animal. Some 

 species possess two eyes, others four or six on the prostomium ; others 

 up to 250 eyes [Amphiporus) arranged in clusters or rows, while the 

 number may vary in different individuals of the same species. The 

 eyes are nearly always subepithelial in type consisting of bipolar cells 

 terminating in a brush border enclosed within a pigment cup of 

 epithehum (Hilton, 1921) (Fig. 93). The eyes of the terrestrial genus, 



1 pp. 724, 733. "^ p. 734. 



Amphiporus 



