I IO S. O. MAST. 



complete in from one to three minutes if the victim is not over 

 twice as large as the Didinium. 



There are at least four factors involved in this process: (i) 

 The pull exerted by the seizing organ; (2) the suction produced 

 by the active expansion of the body; (3) the extension of the oral 

 opening out over the object; (4) the pressure produced by the 

 contraction of the mouth when the object is nearly swallowed. 



1. That the seizing organ travels to the posterior end of 

 Didinium and remains attached for some time after the prey is 

 swallowed was shown by Thon (1905, p. 298) by means of sec- 

 tions. In entire specimens mounted in glycerine it can be seen 

 attached to organisms which extend more than half way down 

 the interior of the body (see Fig. 8). The seizing organ returns 

 to its former position after the food is partially digested. If 

 Didinia are fed on Colpoda, it frequently happens that the 

 ectosarc breaks immediately after the seizing organ is attached 

 to it ; then only a very small portion remains attached to the 

 organ, which at once travels to the posterior end of the body 

 drawing the bit of ectosarc with it. This portion of substance 

 is frequently so small that it does not fill the space left by the in- 

 drawn organ and in such specimens one can clearly see a marked 

 canal extending from the mouth to the posterior end of the 

 animal, as represented in Fig. 18. The presence of this canal 

 in some of the specimens studied by Balbiani led him to conclude 

 that there is a permanent gut in Didininm. It is, however, 

 nothing more than an invagination formed by the movement of 

 the seizing organ and it disappears as soon as this organ returns 

 to its accustomed position. In this canal the food lies. It is 

 therefore in one sense merely in contact with the surface of the 

 body after it is swallowed, much as in Amoeba where the ectosarc 

 surrounds the food particles and is taken in with it. 



2. In some specimens the canal referred to above remains open 

 to the exterior and fills with water as fast as it is formed. In 

 others, however, the oral opening closes as soon as the organ is 

 drawn in and this, of course, prevents the canal from filling. 

 In such specimens, marked depressions are formed in the sides, 

 especially near the anterior end, so that an end view of a 

 Didinium in this condition sometimes presents an outline which 



