The Coelenterata 251 



Hydra obtains its food by throwing out nematocysts and paralyzing 

 its prey. The surface of the tentacle itself is somewhat sticky, which 

 assists in keeping food from getting away, once the tentacle bends about 

 it and carries it to the animal's mouth. After the food enters the mouth, 

 the forepart of the animal contracts to send it downward. 



There are gland-cells in the entoderm which secrete a digestive 

 fluid, and it is probable that some digestion takes place in the entoderm 

 cells themselves. These latter have little flagella by which food is 

 whipped about. When digestion takes place within these entoderm cells, 

 digestion is said to be intracellular. 



It is interesting to note that Hydra will not respond to food stimuli 

 or capture prey after being fed. 



The normal position of Hydra is an attachment to some solid object 

 by its basal disk. When the animal moves from one attached place 

 to another, it uses its tentacles as feet, slowly moving them along as 

 though walking upon them, and when a suitable location has been 

 found, releasing its body at the basal end and attaching it to the newly- 

 found spot. (Fig. 154.) 



The reproduction of Hydra is especially interesting in that it fur- 

 nishes us with excellent proof for Weismann's insistence on the separa- 

 tion of somatoplasm and germ-plasm. 



This animal usually reproduces by budding, as does yeast, except 

 that the bud in this instance pushes out and becomes stalk-shaped. The 

 tentacles of the bud grow from the distal end of the new stalk bud, and 

 the entire new organism is pinched off from the mother stalk or body. 

 (Fig. 152, C.) _ 



In fact, it is not uncommon for one of these buds to form new buds 

 on its body before it is ready for independent existence itself. At all 

 times, the cavity of the newly forming animal is in direct continuation 

 with the mother cavity, until the pinching-off process occurs. 



There is a division of the body sometimes, though very infrequently, 

 by simple fission ( ), that is, by a splitting of the 



entire animal lengthwise, commencing from the distal end and extending 

 to the basal disk. Sometimes, also, even the buds reproduce in this 

 manner, while transverse fission is not unknown. 



In the sexual method, the spermatozoa from the testes escape into 

 the surrounding water. The eggs arise in the ovary from ectodermal 

 interstitial cells. Usually only one egg in the ovary grows to maturity, 

 though several may begin growth, only to have one of them — the 

 stronger by virtue of position, or ability to obtain more food — absorbing 

 the others. Two polar bodies are given off from this egg when it is 

 ready for fertilization, and then it is said to be mature. 



The cleavage of the egg is total, and almost equal. After this origi- 

 nal egg has divided several times, the blastula is formed with a cavity 

 called the blastocoel ( ). Cells from the inner 



portion bud off and make a sort of solid gastrula-like structure; this 



