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 Paramecium 
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 alge, about 
