CCELENTERATA. 



169 



pores. The principal opening serves as a real mouth as well 

 as vent for the voiding of undigested matter, whereas in 

 sponges it is not a mouth in any sense. In general the indi- 

 vidual even in the colonial forms of ccelenterates is more dis- 

 tinctly an individual than in the sponges. The division of 

 labor among the parts and the interdependence of parts is 

 rather greater than among the sponges. 



FIG. 79. 



H. 



"v 



FIG. 79. 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; //., flagellum; g.v., gastro-vascular cavity; m., mouth; mes., mesenchyma 

 (noncellular) ; m.f., muscular processes of the ectodermal cells; n, nettling cells; n', 

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



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

 tinguished 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? 



