178 



ZOOLOGY 



cavity) in which both digestion and circulation occur No 

 coelom. 



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

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



Fig. 81. 



■^ B 



___^ ■■•vt' 

 *l- ^"'^[[^*^'^ 



FzG. 81. At Longitudinal section through the body of Hydra (diagrammatic). B, small 

 portion of the wall more highly magnified, b, bud; ecu, ectoderm; ent., entoderm; /, foot; /^, 

 flagellum; ».»., gastro-vascular cavity; m., mouth; met., mesenchyma (non-cellularl; m./., muscular 

 processes of the ectodermal cells; n, nettling cells; n', same, exploded; na., nucleus; t, tentacle; v, 

 vacuole. 



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

 in Hydra? What differentiations are represented in the ectoderm 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 

 with cells of various kinds, derived from the other layers. 



