THE EYE IN EVOLUTION 



The PLATYHELMiNTHES have sense organs only of the most 

 rudimentary type — if any. The freely-hving turbellarians (Plana- 

 rians, etc.) are the most adequately equipped with eyes (Figs. 168 to 

 170). These may be merely two or four in number, in which case they 

 lie on the dorsal aspect of the head-end associated with the tentacles 

 near the cerebral ganglion, as in the fresh-water Rhabdocoela ; but 

 others such as the marine Polycladida may possess several hundred. 

 A common arrangement, well seen in the Tricladida, is that these 

 multiple ocelli are distributed around the circumference of the body 

 concentrated particularly at the anterior margin (Figs. 168 and 170) 

 (Busch. 1851 ; Hyman, 1938-51). The eyes are always very elemen- 



FiGS. 168 TO 170. — The Eyes of Turbellarian Worms. 



\k s- „ 



M 



Fig. 168. — A land pla- 

 narian, Geoplana mexi- 

 cana. 



There is a row of eyes 

 along the entire margin 

 of the animal (after 

 Hyman). 



Fig. 169.— The eyes of 

 the pelagic Rhabdo- 

 coele, Alaurina proli- 

 fera. 

 S, papillated snout ; 



M, mouth ; E, paired eye 



(after Busch). 



Fig. 170.— The eyes of 

 the fresh-water pla- 

 narian, Polycelis coro- 

 nata. They are concen- 

 trated at the head-end 

 (after Hyman). 



Dendrocoelum 



tary, and lacking a dioptric apparatus are capable only of light 

 perception although a directional appreciation may be evident 

 (Taliaferro, 1920). The number of visual cells is said to vary between 

 1 and 200 (Hesse, 1896 ; Schmidt, 1902). Occasionally, as in 

 Dendroccehim, they are of the flat epithelial type (Fig. 95). Usually 

 they are of the subepithelial type, appearing as minute pigmented 

 spots about 0"1 mm. in diameter and consisting of a pigmented goblet 

 enclosing the sensory cells (Figs. 91 and 92). In these the sensory 

 ce-lr- .T,re of the bipolar type with a striated margin facing away from 

 tlK direction of light to form an inverted retina. When the eyes are 

 nea ''hi^ cerebral ganglion the sensory fibres enter the latter directly ; 

 othc -e they enter the peripheral nerve-net. 



