THE COEI.ENTERATA 



249 



Two kinds of nematocysts smaller than that just described are also 

 found in the ectoderm of Hydra. One of these is cylindrical and con- 

 tains a barbless thread which, when discharged, aids in the capture of 

 prey by coiling around the spines or other structures that may be 

 present. 



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, ab- 

 sorptive, and secretory. The digestive cells are large, with muscle fibrils 

 at their base, and flagella or pseudo- 

 podia at the end which projects into 

 the gastrovascular 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 foun& lying at the 

 base -of the other entoderm cells. 



Fig. 153. Transverse Section of Hydra fusca. 

 1. Ectoderm cells (myo-epithelial) . 2. 

 Interstitial cells. 3. Nematocysts. 4. Coelen- 

 teron. 5. Endoderm cells. 6. Vacuoles. 7. 

 Food granules. 8. Nuclei. (After Shipley 

 and MacBride). 



Fig. 154. 



Hydra moving like a measuring worm and 

 using tentacles as feet. (From Jennings, after 

 Wagner). 



The mesoglea is an extremely thin layer of jelly-like substance situ- 

 ated between the other two layers. 



From recent investigations it seems well established that Hydra 

 possesses a nervous system, though complicated staining methods are 

 necessary to make it visible. In the ectoderm there is a sort of plexus 

 of nerve-cells connected by nerve-fibers with centers in the region of 

 the mouth and foot. Sensory cells in the surface layer of cells serve as 

 external organs of stimulation, and are in direct continuity with fibers 

 from the nerve cells. Some of the nerve-cells send processes to the mus- 

 cle fibers of the epitheliomuscular cells, and are therefore motor in func- 

 tion. No processes from the nerve-cells to the nematocysts have yet 

 been discovered, though they probably occur. The entoderm of the body 

 also contains nerve-cells, but not so many as are present in the ecto- 

 derm. 



