The eyes of monogenetic trematodes are mostly paired in two 

 pairs or more seldom, one pair. Annong the highest Monogenoidea one 

 finds only one eye but of the paired type by its origin. The indication of 

 the presence of six to eight eye spots in (Hexabothrium appendiculatum 

 Kuehn) was erroneous as we have explained before. What was mistaken 

 for eyes proved to be glandular cells. The eyes are located on the dorsal 

 side of the body usually above the pharynx or in front of it closer to the 

 anterior end. For the most part, eyes are of different sizes --the anterior 



Fig. 75. Heteraxine heterocerca 

 (Goto), nervous system of the adult 

 worm. (According to Goto, 1894, 

 simplified). 



Fig. 76. Capsala martinieri Bosc, 

 nervous system of the adult worm. 

 Somewhat diagramnnatized and 

 and simplified. (According to Lang, 

 1880). 



pair is usually smaller than the posterior, however, the reverse relation- 

 ship is also observed. Often the eyes are strongly reduced or even com- 

 pletely disappear. The reduction and degeneration of the eyes can be easily 

 traced in a number of Dactylogyridae especially among representatives of 

 the genus Acolpenteron which are parasites of the ureters of their hosts. 

 A number of fornas have eyes only in the early stages of development. The 

 structure of the eyes is prinaitive enough (Fig. 77). Usually the eye is in- 

 verted and consists of a pigmented globule in the shape of one large cell 

 varying in color from amber to black and consisting mostly of unicellular 

 retinae with a fringe of rods adjoining directly the pigmented globule from 

 its concave side or adjoining a special layer lying between the retinae and 

 the pigmented globule. These rods are usually analogous to the eye rods 

 of higher animals. Among a nunriber of forms there is also a special light 

 refracting little-lens which lies in front of the eye globule. Andre's 



53 



48 



