178 THE INVERTEBRATA 



round nuclei as in the majority of forms. Externally the cells are 

 ciliated, the cilia being arranged in tracts over the surface of the body. 

 Inside the cells are seen a number of crystalline, rod-shaped bodies, 



-Lexc.ca, 



:exc.po. 



Fig. 143. The excretory canal system in Dendrocoelum lacteum. After Wil- 

 helmi. exc.po. excretory pores opening to exterior; l.exc.ca. lateral longi- 

 tudinal main excretory canal. 



known as rhabdites. Although much has been written about rhabdites 

 their function remains obscure. They are a secretion, more or less 

 firm, which dissolves and becomes liquid in contact with water. They 



