CHAPTER 5 



THE EYES OF FLAT WORMS— PLATYHELMINTHES 



In the flat worms, e.g. the fresh-water Planaria or Dendroccelum, there 

 is very evident bilateral symmetry. At the anterior or head-end of the 

 body are a pair of rounded or kidney-shaped black spots — the eyes, or 

 ocelli. They are situated on the dorsal surface of the body and are 



B 



Fig. 54. — Ventral Aspect of Larva of Yungia aurantiaca. 

 A — commencement of metamorphosis. B — Metamorphosis almost completed. 

 In A, two pairs of ocelli have appeared in the region occupied by the apical 

 cells of the earlier embryo (Muller's larva). In B, additional pairs of eye- 

 spots have appeared in the same region. A pair of temporary eye-spots not 

 shown in the figure also appear on the dorsal aspect of A near the base of the 

 ciliated lobe which overhangs the mouth. 



(After Lang, from MacBride.) 



connected by nerve-fibres with the anterior commissure or cerebral gang- 

 lion which joins the right and left longitudinal nerve cords (Fig. 53, B). 

 The eye-spots consist of two or more pigmented epithelial cells which 

 enclose a cup-shaped cavity filled with a clear, refractile substance. 

 Nerve-fibres leave the deep surface of the organ and join the cerebral 



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