146 



AN .INTRODUCTION TO ZOOLOGY 



of water, which enters the incurrent canal, passes through the 

 prosopyles into the radial canal, and thence into the cloacal 

 cavity, finally escaping from the body by way of the osculum. 



The surface area of 

 the epithelium cover- 

 ing the body, and 

 lining the internal 

 cavities, is enormously 

 increased by the canal 

 system. 



Grantia is an animal 

 possessing an outer 

 dermal layer of cells, 

 an inner gastral epi- 

 thelium, and a middle 

 region containing cells 

 of several varieties 

 (Fig. 71). The der- 

 mal epithelium (Fig. 

 71, ect.) covers the 

 entire outer surface of 

 the body, and lines 

 the incurrent canals. 

 It is composed ex- 

 ternally of a single 

 layer of flat cells. 

 Wherever prosopyles 



occur connecting the 

 FIG. 71. Part of the body of Grantia. ect., ecto- incurrent with the 

 derm; end., entoderm ; mes., mesoderm ; , 



5#,spicules. (From Dahlgren and Kepner.) radial canals, a Single 



large dermal cell, 



termed a porocyte, is present. The porocytes are derived from 

 cells of the dermal epithelium. They are large and granular, and 

 frequently exhibit ameboid movements. The prosopyle is an 

 intracellular perforation of the porocyte. Cells, called sclero- 



