PHYLUM CCELENTERATA 



III 



this tube turns rapidly inside out and is able to penetrate the 

 tissues of other animals (Fig. 67, B; Fig. 68, A). The explosion^ 

 is probably due to internal pressure produced by osmosis, and 

 may be brought about by various methods such as the application 

 of a little acetic acid or methyl green. Many animals when 

 " shot " by nematocysts are immediately paralyzed and some- 

 times killed by a poison called hypno- 

 toxin which is injected into it by the 

 tube. 



Two kinds of nematocysts smaller 

 than that just described are also found 

 in the ectoderm of Hydra. One of 

 these is cylindrical and contains a 

 thread without barbs at its base; the 

 other is spherical and contains a barb- 

 less thread which, when discharged, 

 aids in the capture of prey by coiling 

 around the spines or other structures 

 that may be present (Fig. 68, B). 



Certain ectoderm cells of the basal 

 disk of Hydra are glandular and secrete 

 a sticky substance for the attachment 

 of the animal. 



The entoderm, the inner layer of 

 cells, is primarily digestive^ absorptive, 

 and secretory. The digestive cells are 

 large, with muscle fibrils at their base, 

 and flagella or pseudopodia at the 



end which projects into the gas tro vascular cavity. The flagella 

 create currents in the gastrovascular fluid, and the pseudopodia 

 capture solid food particles. The glandular cells are small and 

 without muscle fibrils. Interstitial cells are found lying at the 

 base of the other entoderm cells. 



The mesojgea is an extremely thin layer of jelly-like substance 

 situated between the other two layers. 



FIG. 66. Nematocysts of 

 Hydra before and after dis- 

 charge, cnc, cnidocil ; nem, 

 nematocyst; nu, nucleus of 

 cnidoblast; t, thread-like tube. 

 (From Dahlgren and Kepner, 

 after Schneider.) 



