SYSTEMATIC ZOOLOGY FOR TEACHERS. 2<>1 



ORDER I. HYDRAS OR FRESH WATER POLYPS. 



All small animals, but sufficiently large to be seen by the unaided eye. 

 They possess a cylindrical body, surrounded by from six to eight tentacles, 

 within the center of which is the mouth. This opens into the stomach, through 

 a short gullet. (See fig. 128) The tentacles are provided with roughened, 

 bead-like projections which are arranged spirally. From these are thrown cut 

 threads that are provided with darts by which animals are paralized, then 

 grasped by the tentacles and transferred to the stomach. These darts are 

 easily seen under the microscope. 



Fig. 128. 



4F J 



A 



M X. i 



Hydra, enlarged. A, life size. 



Respiration is doubtless accomplished in the hydra by the absorption of 

 oxygen through all parts of the body wall. 



Digestion also by the absorption of the nutricious portion of the food 

 engulfed in the stomach. 



Propagation is accomplished by two methods ; first, sexually by budding, 

 that is through the accumulation of cells at a certain point on the body wall, 

 a protuberance grows out that ultimately produces a small hydra. When 

 -fully developed, this drops off and leads an independent existence. 



Second, eggs are produced within the body wall which beccme fertilized 

 either by spermetazoa, which are developed by the same individual, or which 

 reach the ova from some other hydra. The eggs, when in proper condition, either 

 burst out through the outer wall or through the inner into the stomach cav- 

 ity. They undergo segmentation and pass into the gastrula stage much as in 

 the sponge. They swim about freely for a time, but finally settle down and 

 produce hydras. 



