REACTIONS OF DIDINIUM NASUTUM. I I I 



is nearly rectangular. This wrinkling of the outer surface indi- 

 cates that there has been an active expansion ; the outer layer of 

 the body has become larger and no longer fits closely over the 

 inner portion. It caves in owing to the decrease in internal pres- 

 sure. During the swallowing process the extension of the outer 

 layer produces a suction which tends to draw substance in. 

 The fact that one frequently finds spaces filled with liquid 

 (water vacuoles) between the object swallowed and the surround- 

 ing wall of the Didinium, also indicates that the internal pressure 

 has been decreased. These water vacuoles can be accounted for 

 by assuming that the decreased internal pressure causes the water 

 to leak in around the object as it is passing the oral edge, or by 

 assuming that it has been forced out of the substance already 

 swallowed, owing to its subjection to a decrease in pressure. It 

 is therefore evident that there is an active expansion in the outer 

 portion of Didinium and that this makes the internal pressure 

 less than the external and produces suction, which tends to draw 

 the food into the body. 



3. That the oral edge moves out to surround the organism 

 captured is evident, but I was not able to ascertain whether this 

 edge creeps out over the body during the swallowing process or 

 whether it is pushed out by the active spreading and extension 

 of the wall back of it. 



4. Finally, direct observation shows that the contraction of 

 the oral opening after the prey is nearly swallowed helps to 

 draw over it the Didinhun, which in this stage of swallowing 

 has the form of a sac. The contraction of the oral opening 

 also forces the prey in at the same time, thus stretching and 

 expanding the thin walls of the Didinium more than would be 

 possible if only the inherent spreading forces within these walls 

 were active. The irregularity in the form of a Didinhun after 

 swallowing a large object supports this conclusion (Fig. 17). 



The most marvelous phenomenon in this whole process is the 

 movement of the seizing organ and the active spreading out of 

 the substance of the Didinium in the form of a sac-like structure. 

 I know of no other organism with the possible exception of 

 Hydra that has the power of such extreme extension. 



One can not work long with this unicellular form without 



