22 Laboratory Guide in Zoology 



the endoderm. True excretion, or the removal of waste 

 material produced by chemical changes within the cells, 

 is performed by the general surface of the body. 



VIII.. — In the ectoderm of the tentacles try to find small, 

 well-defined oval bodies near the margin. They are 

 called nematocysts, are contained within nettle cells, 

 and correspond in . function to the trichocysts of the 

 Paramecium. Within this oval sac there is a long, 

 spirally coiled thread which the Hydra has the power of 

 throwing out to wound its prey. Doubtless some poison 

 ejected with the thread produces the wound. These 

 organs are put to great use in capturing food, though 

 none can be used a second time. You may see the 

 threads by drawing acetic acid under the cover glass. 



IX. — Observe the Hydras in the culture jar once more. 

 Try to find one that has a blunt projection on the side 

 of its body or one that ha.S a small Hydra attached to it. 

 These are doubtless stages in the process of reproduction 

 by the asexual method. There is also another way in 

 which Hydras reproduce their kind — the sexual method. 

 These processes are explained in the following sections. 



X. — Asexual Reproduction. Multiplication by this pro- 

 cess is called budding. A blunt process or bud appears 

 on the Hydra's side, formed at first only of ectodermal 

 cells. This is gradually pushed out and filled with endo- 

 dermal cells. As the bud increases in size, a cavity, 

 continuous with that of the main body cavity of the 

 parent Hydra, is developed within the mass of endo- 

 derm. Later a mouth and tentacles are formed at the 

 outer end of the bud ; its stalk is then constricted at its 

 union with the parent, and the free-swimming young 

 Hydra moves away. Some stages of this process of 

 budding may be easily found by examining all the 

 specimens at hand. 



