190 BIOLOGY OF THE PROTOZOA 



directed downward and the tentacles are stretched out in all direc- 

 tions, forming a forest of plasmic processes among which smaller 

 ciliates, such as Urocentrum turbo, Gastrostyla steinii, etc., or flagel- 

 lates of all kinds may become entangled without injury to them- 

 selves and without disturbing the Actinobolina or drawing out its 

 fatal darts. When, however, an Halteria grandinella, with its quick, 

 jerky movements, approaches the spot, the carnivore is not so 

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

 Halteria 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 slowly 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 Actinobolina 

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



While these observations do not prove that Actinobolina radians 

 eats nothing else, it is certainly true that the usual food is Halteria 

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

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

 tures of Actinobolina 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 1 to 3 dozen individuals 

 of Halteria, grandinella 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 brought about by 

 some specific chemotactic stimulus is responsible for the apparent 

 "choice" of food by Actinobolina, and chemotactic or thigmotactic 

 stimuli for food capture by "circumfluence," " circumvallation " and 

 "importation." 



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

 limitations of their mouth parts. Forms like Didinium, Spathidium, 

 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 innutritious 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 ingestion in many organisms. Thus Actinobolina 

 radians often captures and paralyzes more Halterias than it actually 

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

 18 Halterias, 11 of which were swallowed while a small group of 

 7 were abandoned uneaten, when the Actinobolina swam away. 



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

 stimulus of living food substances, and apparently ignores types 

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

 and Stent or, Metalnikoff and Schaeffer describe an apparent selection 



