CCELENTERATA 



177 



cai'ity) in which digestion and circulation both occur. No 

 ccelom. 



2. Parts radially arranged about an oral-aboral axis. Ten- 

 tacles usually occur at the oral pole (Figs. 82, 85). 



FIG. 81. 



- s 



f~ 



nu. 



PIG. Si. A, T iMj.il >! in ill section through the body of Hydra (ffi^mniinlii) B. cmaB 

 portion of the wall more highly magrrifird. b, bod; ect^ ectodam; cmL. niUxIniu; /. foot; fL. 

 flagellcm; g.i., gastro- vascular cavity; ., moath; met., mesenchyma (noo-ceflnlarl; m./-, ! nlii 

 processes of the ectodennal cells; IB, nettling cells; **, gam**, exploded; mi , mirl^|- f f ^*y^- y 



Questions on the figures. How many cellular layers are to be distinguished 

 in Hydra? What differentiations are represented in the **t**1flrm in different 



regions? In the entoderm? What is the relation of the bud to the adult? Why 

 is the cavity called a gastro-vascular cavity? How is contraction effected in 

 Hydra? 



3. A supporting layer or mass (mesenchyma) between 

 ectoderm and entoderm, sometimes without cells. More often 

 cells of various kinds occur, which have migrated from the 

 other layers. 



12 



