PLATYHELMINTHES 



179 



are formed in special cells, lying either between the ectoderm cells 

 or just beneath them in the parenchyma, and distributed thence to 

 the ectoderm cells. Rhabdites are usually absent from the ectoderm 

 cells in the neighbourhood of sense organs. It will be noticed that 

 when turbellaria are placed in an irritant fluid for preservation such 

 as acetic acid the body is seen to become covered with an opaque white 

 layer. Whether this opaque layer is produced from the rhabdites or 



— - — ■' --• nil. 



-.—---= ci. 



Fig. 144. Terminal organ of an excretory canal, the flame cell. After 

 Wilhelmi. ca. excretory canal; ci. bundle of cilia forming the "flame"; 

 nu. nucleus of flame cell; p. cytoplasm of flame cell. 



from the slime glands which occur in certain regions of the body it is 

 difficult to say. 



Immediately below the ectoderm lies the basement membrane. This 

 is a thin transparent structureless layer, which probably assists in 

 preserving the general shape of the body and serves as an attachment 

 for the muscles which lie immediately beneath it. 



The basement membrane is continuous over the body except where 

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



