BEHAVIOR OF CCELENTERATA 217 



oxygenated regions is probably brought about through a process of 

 trial, the organisms wandering irregularly till they come into oxygen- 

 ated regions and there remaining. If the water is allowed to become 

 very foul, all the Hydras soon collect at the very upper surface, often 

 in contact with the surface film itself. 



Let us now examine the usual behavior of Hydra in obtaining food, 

 as described by Wagner (1905). As we have seen, the undisturbed 

 green Hydra changes its position at intervals, thus in the course of time 

 exploring thoroughly all the region about it. The tentacles of the green 

 Hydra are comparatively short, so that such exploring movements are 

 needed. In the colorless Hydras the tentacles are often excessively long 

 and slender, lying in coils on the bottom, and almost filling the sur- 

 rounding waters with a network of fine 

 threads. They may reach three or 

 four inches in length. In these spe- 

 cies changes of position are less 

 frequent, the great length of the 

 tentacles rendering this unnecessary. 

 When a small animal comes in con- 

 tact with one of the tentacles, in a 

 typical case a somewhat complicated 



reaction OCCUrS. The nematOCVStS of, Fig. 131. — Hydra endeavoring to swal- 

 . . . t • i 1 • 1 l° w a large annelid. Camera drawing. 



the region with which the animal 



comes in contact are shot out, causing the organism to cease its move- 

 ments. The tentacle is viscid and clings to the animal. Now the 

 tentacle is bent toward the mouth. At the same time the other ten- 

 tacles bend in the same direction. If the animal is a large one and is 

 inclined to struggle, the other tentacles seize it, and many nematocysts 

 are shot out and pierce it, so that the organism may become quite 

 covered with these structures. An insect larva which was rescued from 

 a Hydra at this stage is shown in Fig. 132, B. Meanwhile, the mouth 

 becomes widely opened, sometimes before the prey comes in contact 

 with it. When the food reaches the mouth, the tentacles usually release 

 it and are folded slightly back, while the edges of the mouth, or "lips," 

 actively work up over the food, till it is enveloped and passes into the 

 cavity of the body. In this way a Hydra often takes organisms much 

 larger than itself. Figure 131 shows such a case, where a Hydra en- 

 deavored to swallow an annelid that was, at a moderate estimate, fifty 

 times its own bulk. The mouth and body were immensely distended, 

 and the worm was about half enveloped. The Hydra seemed then to 

 have reached its utmost limit, and the process stopped. 



We now wish to analyze this complicated behavior, determining as 



