1. When Didinium comes in contact with an object, it usually responds with the avoiding reaction. In this response it always turns toward the same side, in spite of the fact that it is practically radially symmetrical. 2. It feeds entirely upon living organisms, principally Paramecium, but it has been known to devour also Colpoda, Colpidium, Frontonia and Vorticella. It captures these organisms by accidentally coming in contact with them in swimming about at random. The prey is held by means of the seizing organ, which in some way adheres to the surface when the contact is made. 3. There are no trichocysts discharged by the Didinia, they come from the victims; neither is the seizing organ thrust out at the prey, nor is the prey paralyzed by poison injected through this organ. 4. The trichocysts function as organs of defence. They are discharged in great numbers when the seizing organ becomes fastened to the ectosarc. This forces the Didinium back mechanically and frequently breaks the connection, thus setting the victim free. 5. Well fed Didinia continue to divide for some time without food, becoming smaller and smaller until they are not more than one tenth the size of the original one. 6. Encysted Didinia are frequently found. The chemical composition of the solution in which they live seems to cause them to encyst, rather than lack of food, as maintained by Thon. 7. The apparent choice of food is due to the fact that the seizing organ will adhere to the surface of some organisms and not to that of others. The Didinia come in contact with all sorts of objects in their random swimming and attempt to swallow all of those to which the seizing organ will adhere.