198 



ZOOLOGY 



or by the production of new animals by means of eggs and sperms, 

 sexually. The bud appears on the body as a little knob, some- 

 times more than one coming out on the same hydra. At first the 

 bud is part of the parent animal, the body cavity extending into 

 it. After a short time (usually a few days) the young hydra 

 separates from the old one and begins life anew in another place. 

 This is asexual reproduction. 



The hydra also reproduces by eggs and sperms. These sperms 

 are collected in little groups which usually appear near the free 

 end of the animal, the egg cells developing near the base of the 

 same hydra. Both eggs and sperms grow from the middle layer 

 of the animal. The sperms when ripe are set free in the water, 

 one of them unites with an egg, which is usually still attached to 

 the body of the hydra, and development begins which results in 

 the growth of a new hydra in a new locality. 



Medusa. — The coelenterate animals are a very large group and 

 contain many other animals than the hydra. All of this group of 



animals are found in the 

 water, by far the greatest 

 number of forms living in 

 the ocean. Among the 

 most interesting of all the 

 coelenterates inhabiting the 

 salt water are the jelly fishes 

 or medusae. 



For the study of the Me- 

 dusae^ use Gonionemus pre- 

 served in formol. Why should 

 you call it a "jellyfish"? Notice 

 the shape of the animal, some- 

 what like an umbrella with a 

 short handle. What do you 

 find hanging from the edge of 

 the umbrella? The tentacles 

 of the medusa are provided with stinging cells. Find and describe the mouth 

 at the end of the handle of the umbrella (manubrium). (N.B. A little car- 

 mine or methylene blue dissolved in water may be forced into the mouth of 

 the specimen with a small medicine dropper. This will show clearly the di- 

 gestive cavity and its canals). Notice the course taken by the canals form- 

 ing the digestive tract. Hanging just under the radial canals of the digestive 

 tract are found the reproductive bodies, eggs or sperms. 



^ See Hiinter and Valentine, Manual, page 156. 



Medusa, {Gonionemits murbachii), showing tentacles, 

 mouth, digestive canals, and reproductive bodies. 

 Photographed from the model at the American 

 Museum of Natural History. 



