174 ELEMENTARY ZOOLOGY. 



Hydra may be studied by comparing the figures to be found in 

 several text-books. (See Fig. 46.) 



H 



FIG. 46. A. a longitudinal section through the body of Hydra (diagrammatic). 

 B, small portion of the wall, more highly magnified, b, bud; ect, ectoderm; ent, ento- 

 derm; f. foot; ft, flagellum; g.v., gastro-vascular cavity; m, mouth; mes, mesenchyma 

 (non-cellular); m./., muscular processes of the ectodermal cells; n, nettling cells; ', 

 same, exploded; nu., nucleus; /, tentacle; v, vacuole. 



Questions on the Figure. How many layers of cells are to be 

 made out in the wall of Hydra? In what way does the ectoderm of 

 different regions vary? The entoderm? What is the relation of 

 the bud to the mother? Why is the cavity called a "gastro-vascular" 

 cavity? How is contraction effected in Hydra? What difference 

 of function can you suggest for ectoderm and entoderm? 



