i8o 



THE INVERTEBRATA 



the ectoderm overlying the sense organs. In certain parts of the 

 ectoderm, notably in the pharynx of the Tricladida, the nuclei 

 of the ectoderm cells sink through the basal membrane and its under- 



9en.po. 



ci.pit 



Fig. 145. Planaria, x about 4 From Shipley and MacBride. e. eye; 

 ci.pit, ciliated pits at side of head ; M. mouth at end of protruded pharynx ; 

 ph. outline of the pharynx sheath into which the pharynx can be with- 

 drawn; gen.po. generative pore. 



n.net 



Fig. 146. Transverse section through the outer layer of pharynx of a triclad. 

 Altered from Steinmann. ba.memb. basal membrane ; aVc.m. layer of circular 

 muscles; ect. ectoderm; gl. glands; long.m. layer of longitudinal muscles; 

 n.net. nervous network; nu. nuclei of ectoderm cells; ra.m. radial muscles. 



lying muscle layer and come to lie in the parenchyma attached to the 

 cells by long strands of protoplasm (Fig. 146). In the Trematoda 



