78 THE BEHAVIOR OF PROTOZOA 



appearing as if forming a mere film over its engulfed victim. 

 Nothing seems too large for Didinia to attack. Their 

 powers of digestion seem equal to their voracity; according 

 to Mast a Didinium will digest an ordinary Paramcecium 

 once every three hours. According to Balbiani Didinium 

 actively pursues its prey and when sufficiently near "it 

 begins by casting at it a quantity of bacillary corpuscles 

 which constitute its pharyngeal armature." Coming to 

 closer quarters it thrusts forth its prehensible apparatus 

 into its victim and drags it back toward its mouth. This 

 behavior is referred to by Binet as a "most complicated 

 instance of localization" involving a precise knowledge of 

 the position of the prey at which the Didinium takes a 

 definite aim. 



We have here an instance of how easily one may be de- 

 ceived in interpreting the behavior of lower organisms. 

 There is no evidence that Didinium pursues its prey like a 

 hunter. Mast has shown that it does not discharge tri- 

 chocysts at a distance; in fact it has none to discharge; the 

 loose trichocysts seen when Didinium attacks Paramcecium 

 and which Balbiani thought were shot out by the hunter in- 

 fusorian were derived entirely from the organism attacked. 

 According to Jennings, Dininium reacts in much the same 

 way "not only to objects which may serve as food, but to 

 all sorts of solid bodies. In other words, the process is one 

 of trial of all sorts of conditions. On coming in contact 

 with a solid, Didinium * tries' to pierce and swallow it. 

 If this succeeds, well and good; if it does not, something 

 else is 'tried.' In a culture containing many specimens of 

 Didinium, the author has seen dozens of individuals reacting 

 in this way to the bottom and sides of the glass vessel, ap- 

 parently making persevering efforts to pierce the glass. 

 Others 'try' water plants, or masses of small algae, about 



