134 



THE EYE IN EVOLUTION 



HoBmadipsa 



(Whitman, 1889-93 ; Maier, 1892 ; Hesse, 1897-1902 ; Biitschli, 1921). In the 

 land-leech, Hoemadipsa, the ordinary segmented papillfe more closely resemble 

 eyes since the visual cells are associated with j^igment (Bhatia, 1956). 



In the second place, bipolar cells with a cihate or brush-like 

 receptor and a proximal nerve fibre may similarly migrate into the 

 subepithelial tissues, aggregating into a cluster in association with a 

 mantle of pigment cells. These are seen typically in the leaf-like 

 turbellarian and the ribbon-like nemertine worms (Figs. 91 to 93). In 

 these, the eye consists merely of one or a number of elongated visual 

 cells with a distal ciliated border, the fibrillar terminations of which 

 run proximally to form an optic nerve ; the organ lies under the 



Figs. 91 to 93. — Subepithelial Eyes (after Hesse). 



Fig. 91. — The eye of the turbellarian worm, Planaria torva, consisting of two 

 light cells with cilia (c), nucleus (71) and pigment mantle (p), the whole 

 Ij'ing underneath the epithelium (e). 



r]~o] * I o|V|^"je|fr e 



HZ 



™aaa, 



Fig. '.2. — The eye of the turbellarian 

 '>)rm, Planaria gonocephala. 



Fig. 93. — The eye of the nemertine 

 worm, Drepanophorus. 



