2 2 OSBORN. [Vol. XVIII. 



mouth region. The posterior trunk, however, can be traced in 

 sections posteriorly to a point near the excretory bladders. The 

 two trunks run widely apart, each lying near the margin of the 

 flattened body. The structure of the trunks is wholly fibrous and 

 faintly stainable; no indications of nerve cell bodies or nuclei 

 are visible in them. Cells are, however, recognizable on the sides 

 of the trunks which may perhaps be nerve cells, but I have not 

 been able to see any fibers connecting them with the trunks, or to 

 prove that they are nervous. 



A pair of eyes are distinctly visible in young individuals, 

 but they are absent in old ones. Attention is called to them by 

 Leidy, who thought that they were functional in the fluke while 

 in the mussel, but this cannot be considered as proved and, as I 

 shall later try to show, is perhaps improbable. The eyes are 

 located (Figs. 2, 2b, 3, 4, 28 and 29) at the front of the 

 pharynx between it and the posterior nerve trunk. It is in close 

 contact with the latter. After maturity the eyes gradually dis- 

 appear, and in consequence we find all stages in the process, some 

 with both eyes well-shaped, others with one eye replaced by a 

 mass of disorganized pigment while its mate is still well-shaped, 

 and still others with no eyes. In 52 cases of which I have pre- 

 served specimens or drawings both eyes are present in 29, none in 

 12, the left eye only in 8, the right being absent, and the right only, 

 and the left had disappeared in 3. 



The eye in the best conditions in which I have found it (Fig. 

 29) is a hollow cup of a diameter of 10 /i. The cup is open at 

 one side and at the opposite side is thickened, growing thinner as 

 you pass to.ward the open rim. The opening looks outward, 

 forward and downward (Fig. 29). The wall of the cup is com- 

 posed of minute grains of very dark brown, almost black, pigment, 

 which in the mass looks black. With an oil-immersion lens one 

 can see that these grains are separated and the indications are that 

 they are imbedded in a non-stainable substance. In some cases the 

 outlines of the cup are as distinct as in the drawing, a sharp line 

 inside at the bottom being as distinct as indicated in the figure. 

 I have not been able to recognize any lens in any of my specimens ; 

 one may have been present earlier and already degenerated. On 

 the opposite side of the eye in some cases I have recognized a 



