124 AN INTRODUCTION TO ZOOLOGY 



nucleus around which are embedded sometimes as many as twelve 

 nematocysts each in its own cnidoblast (Fig. 55, A). The cni- 

 docils projecting from the cnidoblasts resemble groups of cilia. 

 Each cnidoblast is drawn out at its base into a contractile fibril 

 which enters the longitudinal muscular sheet at the base of the 

 ectoderm cells. 



ENTODERM (Fig. 54, en.). The inner layer of cells, the ento- 

 derm, occupies about two thirds of the body wall. Its functions 



are digestive and secretory. The di- 

 gestive cells are long and club- 

 shaped, with transverse muscular 

 fibrils at their base, forming a cir- 

 cular sheet of contractile substance. 

 At the larger end, which extends 

 - nto ^ central gastro vascular 

 FIG. 58. Three glandular cells cavity, are two flagella. Pseudo- 

 horn the basal disk of Hy- p dia may also be thrust out from 

 dra ad. sec., _ granules of this f ree en(L The i nter nal struc- 

 adhesive secretion. (From A r , ,.~ , . 



Dahlgren and Kepner.) ture f these Cells dlffers before and 



after the animal is fed. In a starv- 

 ing Hydra large vacuoles appear, almost completely filling the 

 cell, the protoplasm being reduced to a thin layer near the cell 

 wall ; after a meal, however, the cells are gorged with nutritive 

 spheres, many of which, especially the oil globules, migrate into 

 the ectoderm and are stored near the periphery, giving the 

 animal its brown color (104). 



The glandular cells are smaller than the digestive cells, and lack 

 the contractile fibrils at their base. They are broad at the free 

 end, and thin out to a fine filament which ends in a knoblike 

 enlargement when the mesoglea is reached. The gland cells 

 also differ in appearance according to their metabolic activity: 

 some are filled with large vacuoles containing secretory matter, 

 while others, having discharged their secretum, appear crowded 

 with fine granules. Interstitial cells are found lying at the base 

 of the other entoderm cells. 



