OF MICR O- OR CAN ISMS. 45 



Now, do these Acinetidae show any preference of 

 choice among the Infusoria that chance to fall 

 within reach of their tentacles? M. Maupas, who has 

 made an especial study of these organisms had at first 

 admitted this preference in choice. But he afterwards 

 rejected the notion. In 1885, he writes us: " I find 

 quite another explanation of the impunity with which 

 the Coleps hirtus can throw itself upon the terrible 

 suckers of the Podophrys fixa. The stout shell with 

 which this little Infusory is enveloped, serves it as a 

 shield and guards it from the deadly grasp of the Acine- 

 tidae. The Acinetidae do not seize the Coleps because 

 of any dislike of the latter, but because they are un- 

 able to seize them, and their inability results from the 

 peculiar structure of the Coleps' tegumentary en- 

 velope. The Paramecia which also escape unscathed, 

 are similarly provided with a tegument of high resist- 

 ing power, which serves them as a protection in this 

 contingency. The Stylonichia histrio, like all other 

 Stylonichiae, has a very soft tegumentary envelope. 

 They are accordingly seized and devoured by the 

 Acinetidae without difficulty. The detailed knowledge 

 of the differences of structure in the tegumentary en- 

 velopes has caused me to abandon the idea of a pre- 

 ference or dislike in the choice of those victims which 

 serve as food for the Acinetidae. Of the prey that 

 passes by, they catch what they can and not what they 

 want to." 



In a large number of species the prehension of food 

 is preceeded by another stage, the search for food, 

 and in the case of living prey, by its capture. We 

 shall not investigate these phenomena among all the 

 Protozoa, but shall direct our attention especially to 

 the ciliated Infusoria. Their habits are a remarkable 



