GENERAL PHYSIOLOGY 183 



its fatal darts. When, however, an HaUeria grandincllo, with its 

 quick, jerky mo\-ements, approaches the spot, the carnivore is not 

 so peaceful. The tentacles are shot out with unerring aim and 

 the IlaJieria whirls around in a vigorous, but vain, effort to escape, 

 then becomes quiet, with cilia outstretched, perfectly paralyzed. 

 The tentacle with its prey fast attached is then slowl\- retracted 

 until the victim is brought to the body and swallowed with one gulp. 

 Within the short time of twenty minutes I have seen an Actinohohis 

 thus capture and swallow not less than ten HaUerias.'" (Calkins) 



While these observations do not prove that AcUnoboIus radians 

 eats nothing else, it is certainly true that the usual foofl is IlaJferia 

 grandinella, a fact which may account for the rarity of Actinobolus. 

 That it thrives on HaUeria is proved by the fact that isolation cul- 

 tures of Actinobolus have been maintained for a period of eight 

 months and through 375+ generations by division during which the 

 only food supplied was a daily ration of 2 to 3 dozen individuals of 

 HaUeria grandineUa independent pure "mixed" cultures of which, 

 with bacteria, were maintained at the same time. In these cases 

 it is quite probable that the motor response due to some specific 

 chemotactic stimulus is responsible for the apparent "choice" of 

 food by Actinobolus, and chemotactic or thigmotactic stimuli for 

 food capture by "circumfluence," "circumvallation" and "importa- 

 tion." 



A certain degree of selection is forced upon some Protozoa by the 

 limitations of their mouth parts. Forms like Didinium, Spathi- 

 dium, Lionotus, etc., with distensible mouths, can handle organisms 

 of various sizes, but forms like Paramecium, Dileptus, Spirostomum, 

 etc., with small inelastic mouths are constrained to "select" small 

 objects for food. Here there is no apparent choice between nutri- 

 tious and innutritions particles, carmine or indigo granules being 

 taken in with the same initial avidity as bacteria or other useful 

 foods. A certain so-called "hunger-satisfaction," however, leads 

 to the cessation of food-taking in many organisms. Thus Actino- 

 bolns radians often captures and paralyzes more HaUenas than it 

 actually eats; on one occasion, for example, an individual was seen 

 to catch 18 HaUerias, 11 of which were swallowed while a small 

 group of 7 were abandoned uneaten, when the Actinobolus swam 

 away. 



Amoeba proteus, after a period of eating no longer reacts to the 

 stimulus of living food substances, and apparently ignores tj'pes 

 which were previously engulfed (Schaefter) . So, too, in Paramecium 

 and Stentor, INIetalnikoff and Schaefter describe an apparent selection 

 of food as illustrated by the rejection of carmine grainiles after a 

 period during which such granules were actually taken in. It seems 

 probable that such phenomena indicate a type of fatigue involving 

 the temporary loss of irritability through which the organism 



