THE EYES OF FLAT WORMS 85 



ganglion. In some marine forms the eyes are multiple and are found on 

 the dorsal aspect of the head, arranged round its anterior margin and sides. 

 In others both marginal and paired eyes are present in the same individual 

 (Fig. 19, p. 25). 



In the liver-fluke, Fasciolia hepaticum, which is sometimes found in 

 the larger bile-ducts of the sheep, no eyes are found in the adult form. 

 This is what might be expected during the parasitic phase of the animal's 

 existence, and it is interesting to note that in its free-swimming larval 

 stage, two pigmented eyes are present and that remnants of these eye- 

 spots are still found in the sporocyst stage of its existence in the body of 

 a snail. 



Eye-spots are also present in the region of the embryonic apical cells 

 on the larva of Yungia aurantiaca (Fig. 51). Two pairs appear first on 

 the ventral aspect, near the median plane ; later, at the period of meta- 

 morphosis, other pairs appear farther forward and situated more laterally. 



Well-developed paired eyes are present in the Triclad forms of 

 Turbellaria, and an eye-spot is found in the anterior segment of Micro- 

 stomium, a simple type of Turbellaria which reproduces by an asexual 

 process of budding. 



In the tape worms eyes are completely absent, these animals being 

 endoparasitic in all stages of their existence. 



