HYDRA FUSCA 145 



that this spiral coiling is to enable the animal to adhere to the 

 minute spines or hairs of its prey by coiling around them in a 

 corkscrew fashion. 



The nematocysts are scattered all over the body of Hydra 

 except in its base. In some parts, especially in the tentacles, 

 they are grouped into little bunches which project from the side 

 and form tubercles; Fig. J. These little clusters are spoken of 

 as batteries. 



The Basal Disk. The base of Hydra is different from the 

 rest of the body. It secretes a sticky substance by means of 

 which the animal attaches itself to an object. This base has 

 the power of causing the animal to glide very slowly over the 

 object upon which it is attached, though the exact method by 

 which this motion is produced is not known. In this part of the 

 body the nematocysts are lacking, and the epithelio-muscle cells 

 not only have muscle fibers but some of them have the func- 

 tion of secreting a cement, and differ in appearance from those 

 of the rest of the body; Fig. K. 



Endoderm. The endoderm is about twice as thick as the 

 ectoderm and contains cells of two kinds, known as the digestive 

 cells and the secretory cells. The digestive cells are long and 

 cup-shaped, and have, extending from their base next to the 

 mesoglcea, fibers of contractile substance. At their inner or 

 free end they bear two lashing flagella; Fig. F. It is interest- 

 ing to note that the free end of these cells may be protruded in 

 the form of pseudopodia, much like those already seen in the 

 Amoeba, and that they are able to take into their bodies small 

 solid particles of food which are then probably digested within the 

 cells of the body itself. Thus Hydra has a function of digestion 

 similar to that of the Amoeba, being able, to a certain extent, 

 to take inside of its digesting cells solid particles of food and to 

 digest them (intracellular digestion). The chief digestion, how- 

 ever, is carried on by the other cells, the secretory cells. These 

 are smaller than the digestive cells and lack the contractile 

 fibers at their base. They produce a secretion which is discharged 



