EYE-SPOTS IN DIGENEA. 



121 



saucer-shape to deeply pocketed (C. gigas). In all cases recorded 

 for the Digenea the opening of the cup is toward the source of 

 light. Thus the organ may function as a light-percipient mechan- 

 ism, although its function is undoubtedly impaired by the position 



FIG. 2. Section through young Cercaria pellucida showing relation of pigment 

 cup to nerve ending in the paired lateral eyes, br, brain center; I, "lens" of 

 authors; op, optic cell; opn, optic nerve. X 660. 



of the nerve fiber in front of the "lens." This type of eye-spot 

 shows no differentiation into rod and cone structure, such as 

 Andre (1910) has depicted for Polystomum integer rimum. The 

 mechanism at its best is decidedly an inferior one. 



DEVELOPMENT. 



The pigment eye-spot of the Digenea, as studied by the writer 

 in Cercaria pellucida Faust, C. urbanensis Cort and C. gigas 

 Faust, presents significant points in development. At an early 

 stage of the germ ball it becomes differentiated in accoi dance 

 with the precocious development of the nervous system in the 

 group. A branch of the posterior dorsalis, with a single nucleus, 

 pushes out from the nerve center to the dorsolateral margin of the 

 embryo. As it reaches a position near the surface somewhat 

 posterior to the cerebral mass, the ectodermal layer of the embryo 

 pushes inward, just posterior to the nerve, so that a pocket is 

 formed with the opening opposed to the nerve cell. The nerve 

 fiber and enlarged end then twist around the inner wall of the 



