PLATYHELMINTHES 



175 



where they may take the form of eyes, otocysts, tentacles and ciUated 

 pits in the ectoderm. They may also occur in the free stages in the life 

 history of the Trematoda and Cestoda. The eyes occur on the dorsal 



ce.ga.-/-- 



9— 



Fig. 141. The nervous system of Acoela, to show the network. After 

 Steinmann. ce.ga. cerebral ganglion (brain); M. mouth; ^ and 2, male and 

 female openings respectively. 



surface where they are visible as dark spots. The retina is formed of 

 cup-shaped cells, which are heavily pigmented. The interior of the 

 cup is filled with special nerve cells, varying in number from two 



