Il8 ERNEST CARROLL FAUST. 



Diplodiscus subclavatus (Goeze), together with an occasional 

 record of the pigment-spots in miracidia these observers showed 

 the rather wide distribution of this type of sense organ in the 

 group. 



In his second memoir de Filippi (1857: 433) described the pig- 

 ment spots of Cercaria echinocerca. Among the granules, he 

 wrote, were two large pockets of irregular form but symmetrical 

 arrangement on the sides of the esophagus, comparable to the 

 eyes of the amphistome, while within the pigment pocket was a 

 nucleus which represented a veritable rudimentary lens. Leuck- 

 art (1863: 465) expressed his credulity about the true lenticular 

 function of the nucleus, but saw the relationship to the eye- 

 spots of ectoparasitic trematodes. He mentioned the occasional 

 presence of such organs in the distome, even in the definitive 

 host. Later (1886: 26) he emphasized their homology to the 

 eyes of the Monogenea. 



Accounts of the eye structure in ectoparasitic trematodes 

 have been given by Lang (1880), Goto (1894), Hesse (1897), 

 Beers (1901), and Andre (1910). Lang worked out the his- 

 tology of this organ carefully for Tristomum coccineum. He 

 found the roots to be a branch of the weak posterior dorsalis, 

 the eye itself to consist of (i) a saucer-shaped pigment cap, (2) a 

 spherical or oval lens, (3) a retinal ganglion cell, and (4) muscle 

 cells. Goto (p. 81) was unable to believe that the eyes of his 

 species were functional because (i) of the position of the pigment 

 granules between the source of light and the lens and (2) because 

 of the degeneracy in part of the lens itself. He was inclined to 

 consider them as temperature sense organs. 



In very young cercariae of Diplodiscus subclavatus Looss 

 (1892: 165) found two or three limpid cells with refractive nuclei 

 situated on each side of the esophagus. He thought these to be 

 connected with the developing eye-spots, "die oberste vorzugs- 

 weise den lichtbrechenden, die unteren vielleicht die per- 

 cipirenden Apparate darstellen. Alle drei bleiben aber noch ein- 

 fache Zellen." Osborn (1903), in connection with the histology 

 of Cotylaspis insignis, described eyes for the adults of this spe- 

 cies as "located at the front of the pharynx between it and the 

 posterior nerve trunk. . . in close contact with the latter." Each 



