PLATYHELMINTHES 



205 



The basement membrane is continuous over the body except where 

 it is penetrated by the openings of gland cells. It is absent beneath 

 the ectoderm overlying the sensory areas. 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- 

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

 cells by long strands of protoplasm (Fig. 151). In the Trematoda 



y/^ii>H n.net. 



Fig. 151. Transverse section through the outer layer of pharynx of a triclad. 

 Altered from Steinmann. ba.mefnb. basal membrane ; circ.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. 



and the Cestoda, the ectoderm cells have all sunk into the paren- 

 chyma, and the body is covered by a thick cuticle secreted by the 

 ectoderm cells. 



The parenchyma (also called the^mesenchyme), which fills the interior 

 of the body, is of very different structure in different Platyhelminthes. 

 It is generally formed of cells with long irregular processes and much 

 intercellular space. Within these cells are small granules and particles, 

 which stain readily. Their appearance and number vary according to 

 the state of health of the animal, whether it is starved or fed, and they 



