184 BIOLOGY OF THE PROTOZOA 



responds to stimuli produced by the chemical make-up of foreign 

 substances, a period of rest being necessary for the restoration of 

 this form of irritability. Selection in another sense, however, is 

 quite important. All kinds of food substances are not equally suit- 

 able for Protozoa any more than they are for individual men. This 

 may be due to the fact that digestive fluids of a given type of ciliate 

 or rhizopod are not adequate to dissolve all kinds of protein; or 

 it may be due to deleterious substances in the protoplasm of the 

 prey. All observers who have attempted to raise Protozoa in pure 

 cultures are familiar with the difficulty of providing the proper food 

 materials and excluding the harmful. Unsuccessful culture experi- 

 ments indicate that these conditions have not been met. Further- 

 more, a culture medium is suitable only when the organism under 

 cultivation continues to live during all phases of its life cycle. 

 The failure of Calkins (1912) to rear a single exconjugant of Bleph- 

 arisma widulans, or of Baitsell (1912) to raise exconjugants of 

 Stylonychia ptistvlata are cases in point. The difficulties encoun- 

 tered in attempts to cultivate Spirostoimnn ambiguum or Stenior 

 coenileus are probably due to failure to find a suitable food or oxygen 

 medium.* In some cases it is quite probable that a variety of 

 proteins is necessary for the best cultural results. Hargitt and 

 Fray (1917) and Phillips (1922) have shown that Paramecium will 

 live on pure cultures of bacteria, but for acti^'e development they 

 found that mixed pure cidtures of certain types of bacteria give 

 the best results. 



Apparent selection of foreign objects used in shell building may 

 be due to the physical consistency of the protoplasm and to its 

 ability to pick up foreign bodies like sand crystals, diatom shells, 

 etc., or in part to the size of the shell-opening through which such 

 objects must pass for storage in the protoplasm. Mud and other 

 fine particles of inorganic matter, like carmine granules, are engulfed 

 with bacteria and other microorganisms which produce the stimulus 

 necessary for the operation of food-taking. After the useful sub- 

 stances are digested the residue, like castings of worms, may be 

 voided to the outside or they may serve a useful purpose in the 

 construction of shells. Rhumbler (1898) was thus able to cause 

 DiffiiKjia to build its shell of finely-ground colored glass. 



A special kind of holozoic food-getting is illustrated by the Suc- 

 toria which, instead of cilia, are provided with suctorial tentacles 

 (Fig. 91). The prey, usually some form of ciliated Protozoa, comes 

 in contact with one of these tentacles and is paralyzed through the 

 action of some kind of poison contained in it. The cortex of the 

 prey is perforated by the end of the tentacle and the fluid endoplasm 

 is sucked into the body of the captor, a stream of granules being 



* See note, page 25. 



