S3] LIFE HISTOR Y OF TREMA TODES—FA UST 53 



pigmentation is present in the subintegumentary areas and follows with pre- 

 cision all the ramifications even to the nerve endings. Figure 37 shows the 

 nerve endings in the anterolateral reaches of Cercaria pellucida. 



In the paired eye-spot a definite subspherical "lens" is found in the region 

 of the concentration of the pigment. In the median eye of the trioculate type 

 a "lens" is sometimes present. The origin of the eye is simple. At an early 

 stage in the germ ball (60 ^l to 80 ij), when the cell masses of the nerve trunks 

 are definitely outlined as they emerge from the ganglion center (Fig. 36), a 

 branch of the posterior dorsalis, larger than any other nerve, pushes out 

 obHquely. It meets an invagination of the ectodermal layer, which may be 

 considered an "optic cup." There soon forms in the hollow of the cup a 

 group of dark brown refractory granules, which entirely lines the optic 

 cup several layers deep and leave but a small cavity within. The optic branch 

 of the dorsalis bends back into the cavity at the place where it first comes in 

 contact with the cup and here ends in a pyriform enlargement, the nerve cell. 

 The structure of this end organ is such that it might be interpreted as a " lens, " 

 if the connection with the nerve is not made out. Cort (1915:15) has placed 

 that sort of an interpretation on the eye structure of C. tirbanensis and C. 

 inhabilis. For the former he states that "each true eye is formed by a mass 

 of pigment in the form of a cup, the bottom of which is thicker than the sides. 

 A lens fits into the opening of the cup, leaving a space between its lower sur- 

 face and the bottom of the cup." And again, for C. inhabilis, "the large 

 eyespots . . . are composed of the lens and the cone of pigment like those 

 already described for the monostome, Cercaria urbanensis." The writer has 

 studied some of Cort's material and has found sections where such an inter- 

 pretation might be made from a single section. But in the preceding or fol- 

 lowing section the connection of this "lens " with the optic nerve is plainly seen. 



The eye structure as studied in this monostome is similar to that found 

 in the Monogenea, especially the type in the posterior eye of Tristomum molae 

 (Hesse, 1897:559; Taf. 28, Fig. 29). 



The nervous system of the monogenetic trematode was first studied in 

 detail by Lang (1880), who made out the pigment cup, a refractory body 

 (lichtbrechender Korper), a ganglion cell, the retina, and eye muscles (p. 41; 

 Taf. I, Fig. 2, Taf. II, Fig. 2). Most later investigators mention only theper- 

 ceptory body and the accessory apparatus, altho Andre (1910:217) has iden- 

 tified the muscle fibers of Lang. The writer believes that the movement of 

 the eyes depends largely on the general bodily movement. 



In the adult Monogenea studied the pigment cup is found to lie between 

 the refractory bulb of the eye and the possible source of light (Goto, 1894:81). 

 In the monostomes, as in Dendrocoelium lacteum (Hesse, 1897, Taf. 27, Fig. 

 10), the hollow of the cup is directed outward, so that light falling on the eye 

 must pass thru the end organ before reaching the inner portion of the pigment 

 complex. 



The pigment of the organism is probably the waste product in the metabolic 

 economy of the worm. Its close association with the nerve endings in the 



