LIFE HISTORY OF TWO RARE CILIATES . 393 



material. The maintenance of a certain ratio between the two 

 masses, making possible a normal interchange between them 

 of material, seems to me to explain the greatly elongated nuclei 

 found in Spathidium and Actinobolus. 



C. Food habits 



Frequent observations of the feeding habits of Actinobolus 

 and Spathidium are suggestive of many interesting questions. 

 In these two forms we have organisms subsisting exclusively 

 so far as we know, upon a special type of ciliate. Actinobolus, 

 moored by means of its oral tentacles, awaits the coming of 

 its victim, Halteria grandinella, before making use of its weapons 

 of offense, Spathidium, a predatory form, swims actively through 

 the water, its anterior end in constant motion, passing with 

 seeming indifference all food material except the little ciliate, 

 Colpidium colpoda. This behavior on the part of the organism 

 would seem to indicate an apparent exercise of choice, but when 

 one comes to a careful analysis of the basis of this choice, one 

 finds himself becoming involved in questions .upon which the 

 observations of many other workers have thrown but little 

 hght. 



Interest in the feeding-habits of unicellular organisms, result- 

 ing in practical experimentation dates back to the time of Gleichen 

 in the latter half of the eighteenth century. The question 

 which arose in the minds of these early observers in the field 

 of protozoology was one which has been discussed by almost 

 every worker on the protozoa since that time, namely: Have 

 these simple forms of animal life the power of selection in the 

 kind of food-material upon which they live, — if so, upon what is 

 the choice based? Many of the experiments of Gleichen show- 

 ing that large quantities of finely powdered carmine or indigo 

 mixed with the water, were taken into the body of these uni- 

 cellular forms, were repeated and confirmed by Ehrenberg in 

 1838. Stein, Entz and many other workers, however, either 

 forgot or ignored these early results, since they expressed the 

 concensus of opinion at that time that infusoria showed a de- 

 cided power of selection, inasmuch as from the mass of sub- 



