STUDIES ON THE ECTOPAEASITIC TRBMATODES OF JAPAN. gl 



homogeneous body capped on the dorsal side by a mass of black pig- 

 ment granules. Immediately beside it there is a tolerably large 

 ganglionic cell, the processes of which pass on to both the dorsal 

 and ventral sides of the lens. In the posterior eye the place of the 

 lens is taken by a large multipolar ganghonic cell also capped by a 

 mass of jjigment granules. The nucleus of this cell is vei-y large and 

 spherical in form ; it stains more deeply than the protoplasm, and 

 no definite structure can be observed Avithin it except some faintly 

 staining granules (Pi. XXIV, fig. 4). 



In the above description I have adopted the name commonly 

 used and have called the structures described eyes. Morphologically 

 speaking they are certainly degenerated eyes ,• and have probably been 

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

 not think that they are functional. In the first place, the pigment 

 granules are situated on the dorsal side and thus prevent the light 

 fi'om reaching the lens, since the dorsal side is the only direction from 

 which light can come. In the second place, there is not always a 

 distinct retina. In Tristomum molae, the species studied by Lang, 

 the retina is said to be present ; but in Trist. ovale 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. 7/ these " eyes " are really still useful to 

 the animal, they may possibly be a temperature sense organ ; and for 

 this purpose their structure seems to answer well. For, the black 

 pigment granules would easily absorb the dark heat-rays and cause 

 some molecular change in the lens which they cap. This lens shows 

 no cellular structure in the anterior eye, but in the posterior eye it is 

 a veritable ganglionic cell, as has already been desciibed. The tem- 

 perature sense organ may be of use to the animal in warning it from 

 wandering too near the extremities of the body of the host, where it 



