132 CALIFORNIA ACADEMY OF SCIENCES. ■ [Proc. 3D Ser. 



While the eyes of all the species of the genus E-pidella 

 are constructed upon the same general plan, Goto in his ex- 

 cellent paper on the Japanese ectoparasitic trematodes calls 

 attention to the fact that in E. ovata they are useless as 

 visual organs. "Morphologically speaking they are cer- 

 tainly degenerate eyes, and have probably been derived 

 from some such eyes as are found in the Turbellaria; but 

 I do not think they are functional. In the first place, the 

 pigment granules are situated on the dorsal side and thus 

 prevent the light from reaching the lens, since it is the only 

 direction from which the light can come. In the second 

 place, there is not always a distinct retina. In Tristomum 

 mola, the species studied by Lang, the retina is said to be 

 present; but in Tristomum ovata there is none, since the 

 ganglionic cells in the immediate vicinity of the lens already 

 mentioned are not in such a position as to receive the light 

 that has passed through the lens." 



In the case of E. squamula, living on the underside of a 

 fish which rests on the bottom for the greater part of its 

 time, the pigment on the anterior side of the lens is placed 

 in a most favorable position. As already mentioned, the 

 eyes are located on the dorsal side of the brain. The tis- 

 sues between it and the exterior are clear, and the light pass- 

 ing in under the fish must of necessity strike the lens and 

 affect the retina, which is always present. 



Furthermore, Goto considers the eye muscles to be 

 merely dorsoventral muscle fibers similar to those found in 

 all parts of the body. Accordingly the eyes are passive, 

 " the worm having no power to direct its eyes in any spe- 

 cial direction." It may be true that these eye muscles are 

 the homologue of the dorsoventral bands, but in E. squa- 

 m-ula the movements they produce, usually simultaneous, 

 cause a definite rotation of the eye, and appear to me to be 

 sufficiently specialized to be considered true eye muscles. 



