i6o GENERAL BIOLOGY 



This very simple plan of structure is indicated in the accom- 

 panying diagram (fig. loi). 



The body of the hydra is too large for the satisfactory 

 study of the form and relations of its constituent cells in the 

 living animal. The two cell layers are easily made out, and, 

 in the slender and transparent tip of an extended tentacle, 

 some large nettling cells (nematocytes) are readily observ- 

 able. These contain transparent ovoid sacs {nematocysts) 

 (fig. 102) occupying slight elevations of the ectoderm of 

 the tentacle. Inside each nematocyst is a closely coiled 

 stinging filament, which when discharged in contact with a 

 waterflea, paralyzes it and prevents its escape. Close be- 

 side each nematocyst is an erect minute sensory point, 

 which when touched by the water-flea, communicates the 

 stimulus that causes the discharge of the filament. This 

 discharge may be caused artificially, while one is watch- 

 ing, if some irritating fluid, like dilute acetic acid, be 

 run under the coverglass that confines the hydra upon 

 a slide. By keeping one's eye at the microscope while 

 the acid is diffusing under the cover until it comes in 

 contact, some of the nematocysts will be seen to be thrown 

 out bodily, while others will throw out only their filaments, 

 which will then hang like long flexuous transparent hairs to 

 the side of the tentacle. 



The cellular structure of the hydra is made out by the 

 study of thin sections and of macerated preparations of dis- 

 sociated cells. In a cross section of the body, ectoderm and 

 endoderm stand out with diagrammatic clearness. Their 

 constituent cells are seen to be considerably differentiated, 

 those of the endoderm being rather larger, and variously 

 shaped at their free internal ends, where some bear flagella, 

 and others pseudopodia. Digestion in the hydra combines 

 the methods of the lower animals hitherto studied, in which 

 the food is engulfed by the protoplasm and digested within 



