CCELENTERATA. 



i6 9 



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- 



f - x 



H. 



-vnf. 



FIG. 79. A, Longitudinal section through the body ofHydra (diagrammatic). B, 

 small portion of the wall more highly, fagnifie.l. b, bud; ect., ectoderm: 

 derm; f, foot; fl., flagellum; g.v., gastio-vascular cavity; m., moutnjTn^^rmesenchyma 

 (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. Ho\v many cellular layers are to be dis- 

 tinguished in Hydra? What differentiations -^ire represented in the 

 ectoderm in different regions ? In the entoderm ? \V"hat is the relation of 

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

 How is contraction effected in Hydra? 



