THE ANIMAL KINGDOM 391 



cells. The thread of a penetrant type of nematocyst is capable of 

 piercing the integument of small animals. In the volvent type 

 the thread forms a coil about objects when discharged. In a 

 third type, the glutinant type, the discharged thread adheres to 

 objects by means of a sticky secretion. All three types are 

 probably used in capturing food (Fig. 220). 



The nerve cells of Hydra are in the form of a loose network 

 extending throughout the ectoderm and also in the endoderm. 

 In medusoid forms there is a nerve ring in the outer rim of the 

 umbrella, which also contains sense organs (simple eyes or pig- 

 ment spots, with or without a lens). 



Hydra attaches itself by a basal disk to the substratum by 

 means of an adhesive substance secreted by the ectodermal 

 cells. In Hydra oligaetis, these cells, under certain conditions, 

 secrete a gas, which, confined in a bubble of mucus, serves as 

 a float from which the hydra hangs downward in the water 

 (Fig. 221). 



The mouth of the hydra is a star-shaped opening in a rounded 

 elevation, the hypostome, about which the tentacles are arranged. 

 The tentacles with the aid of the nematocysts capture food and 

 bring it to the mouth. The single layer of cells composing the 

 endoderm is thicker than the ectoderm. Its principal cells are 

 epitheliomuscular cells and gland cells. Most of the epithelio- 

 muscular cells of the endoderm bear several flagella on their 

 free surfaces. They are digestive in function, ingesting and 

 digesting food after the manner of an amoeba (intracellular 

 digestion). The gland cells are also flagellated but are without 

 contractile fibers at their bases. They secrete a digestive fluid, 

 by means of which food is digested in the gastrovascular cavity 

 (extracellular digestion). Undigested material is ejected through 

 the mouth. The peristomal (about the mouth) gland cells and the 

 gland cells of the basal-disk region differ in appearance from 

 the remainder and may have special functions. Interstitial cells 

 are also found in small numbers in the endoderm near the mes- 

 oglea. A few nerve cells also occur in the endoderm. 



The contractile fibers of the endodermal epitheliomuscular 

 cells run transversely to the main body axis of the hydra and thus 

 form a circular muscular band. This, with the longitudinal band 

 formed by the fibers of the ectodermal epitheliomuscular cells, 

 accounts for the active and varied movements of the hydra- 



