HYDRA 161 



is a hollow projection of the body-wall and has the same 

 structure. 



Among the ectoderm cells of the distal portion of the body, 

 and especially of the tentacles, notice the oval, highly refractive 

 stinging organs or nematocysts. Each one consists of a spiral 

 thread-like tube, with several barbs at its base, which lies coiled 

 within the cavity of a cell called a cnidoblast. The cavity is 

 filled with a poisonous fluid; its walls form an ovoid sac, of 

 which the tube is the very much elongated and invaginated 

 outer end. A minute spine projects beyond the free surface 

 of the cnidoblast into the water. When the surface of the 

 ectoderm is irritated the tube is evaginated and shot violently 

 out, and the poisonous fluid contained in the cavity of the 

 nematocyst is injected into any animal that may be struck. 



Exercise 2. Make a large semidiagrammatic drawing of the 

 animal showing the details of its structure; label all 

 carefully. 



Methods of reproduction. Hydra reproduces both sexually and 

 asexually. Well-fed polyps will soon begin to bud off new 

 individuals. The bud makes its appearance first as a projec- 

 tion of the body-wall, and soon becomes a distinct branch. 

 Tentacles and a mouth make their appearance at the extremity 

 of the branch, and the young polyp is complete. It remains 

 attached to the parent for a while, then detaches itself and 

 begins an independent life. 



Besides this asexual method of reproduction, which may go 

 on as long as the animal is well fed and vigorous, reproduction 

 by sexual methods also occurs at more or less irregular inter- 

 vals. Sexual organs appear in the form of projections of the 

 ectoderm of the body-wall. Two classes of these projections 

 appear, smaller ones, which are testes, and larger ones, which 

 are ovaries. The former of these organs, which lie near the 

 distal end of the animal, produce spermatozoa; the latter are 



