GENERAL PHYSIOLOGY 191 



of food as illustrated by the rejection of carmine granules 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 

 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. 



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. 



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

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

 (Fig. 100). 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 

 visible within the tentacle. In some cases it is said that the endo- 

 plasm of the captor flows through the tentacle and into the body 

 substance of the prey where the latter is digested (Maupas, 1883). 

 The body of the victim gradually collapses until nothing remains 

 but the denser walls and the insoluble parts. 



Many of the Protozoa, while parasitic in the cavities and cells of 

 different animals, retain the holozoic method of food-getting, feed- 

 ing upon parts of the protoplasm of the host or upon other living 

 organisms such as bacteria of the digestive tract, or solid detritus 

 of one kind or another. Thus Endamoeba coll lives on intestinal 



