STUDIES ON THE ECTOPARASITIC TEEMATODES OF JAPAN. g^ 



liomoixeneons liody capped on the dorsal side l)y a mass of black pig- 

 ment «crannies. Immediately beside it there is a tolerably large 

 ""ano'lionic cell, the processes of which pass on to both tlie dorsal 

 and ventral sides of the lens. In the posterior eye the pl:iceofthe 

 lens is taken l)y a large mnlti polar ganglionic cell also capped l)y a 

 mass of ])igmerit grannies. The nuclens of this cell is very large and 

 spherical in form ; it stains more deeply than the ])ri)toplasm, and 

 no definite stiaictnre can be observed Avilhin it except some faintlv 

 staining grannies (PI. XXIV, hg. 4). 



In tlie al)ove description I have adopted the name commonly 

 used and have called the strnctnres described eyes. ]\Iorphol(\gically 

 speaking thev are certainly degenerated eyes ; and have probaljly been 

 derived from some sncli eyes as are fonnd in the Tnrbellaria ; but I do 

 not think that thev are functional. In the hrst place, the pigment 

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

 from 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 Tristoiiuini molac, the species studied by Lang, 

 the retina is said to be present ; but in Trist, ovah' there is none, since 

 the ganglionic cells in the immediate vicinity of the lens already 

 mentioned a.re not in such a position as to receive the light that has 

 ])assed through the lens. Jf these " eyes " are really still useful to 

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

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

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

 some molecnlar cliange in the lens which they cap. This lens shoAvs 

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

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

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

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



