322 NA TURE-STUD Y RE VIEW [10-^— Nov., 1914 



animal. If the animal is attached to the side of the jar it will be 

 necessary to loosen its hold by shoving it off with the tip of the 

 pipette, when it may be drawn up as before and transferred to the 

 tumbler. The tumbler may be filled half full of water from the 

 larger jars or from the tap. 



When you disturb hydra in this process it contracts into an 

 almost spherical mass, the tentacles are now simply little promi- 

 nences on one side of the sphere. In fact it may contract so much 

 that you will fail to see it in the tumbler unless you look very 

 sharply for the altered form. It will shortly re-establish itself on 

 the side of the tumbler or the surface film, expand and await some 

 chance animal as food. 



You will undoubtedly have brought in with the material on 

 which hydra was found an abundance of small animals that will 

 serve it as food. Small crustaceans will be observed swimming 

 about in the water. Some of these collect in clouds on the surface 

 of the water on the side of the fruit jar near the light. Here, too, 

 you are likely to find the blood work Chironomus which is really 

 the larva of one of the tiny gnats. Animals of this sort may be 

 transferred to the tumbler in which hydra is living to feed it. It 

 is interesting to watch this process of feeding and to do this it is 

 well to transfer the hydra in a drop of water to a piece of glass. 

 If the drop with the hydra in it is ejected from the pipette carefully 

 the drop or two will make a hemispherical heap of water on the 

 clean surface of the glass and in this drop the hydra will shortly 

 expand ready for a meal. Now, if two or three of these small 

 animals can be added to the drop from the pipette taken up in as 

 little water as possible, the animals are confined in a small territory 

 and soon they will bump up against the tentacles of hydra. If 

 the hydra has been kept for a couple of days in the tumbler without 

 food, it will probably be hungry enough to feed quite promptly. 

 The blood worm for instance comes in contact with hydra's ten- 

 tacles and seems to be shocked by the contact. It remains quiet 

 for just a moment and then as it begins to wriggle the other ten- 

 tacles move over in its direction and several of them lay hold on it. 

 As a matter of fact these tentacles of hydra are provided with 

 thousands of nettle cells. Each one of these cells is furnished with 

 a sharp thread which can be ejected with some force and which 

 penetrates the skin of the blood worm. This thread is really a 

 fine tube and through it a poisonous fluid is sent into the captured 



