PHYLUM PROTOZOA. CILIATES 



57 



specimens in the form of a blister. Under 

 the higher powers of the microscope the 

 pelhcle is then seen to be made up of a great 

 number of hexagonal areas produced by 

 ridges on the surface (Fig. 26). 



The distribution of the motile organelles, 

 the cilia, corresponds to the arrangement of 

 the striations on the pellicle, since one 

 cilium projects from the center of each 

 hexagonal area (Fig. 26). These hairlike 

 structures occur on all parts of the body, 

 those at the posterior end being slightlv 

 longer than elsewhere. Cilia are outgrowths 

 of the cell protoplasm and arise from basal 

 bodies. The arrangement of the cilia within 

 the cytophar}'nx is rather complicated; they 

 guide the food particles that are swept 

 within their reach. 



Physiology 



Physiologic processes similar to those de- 

 scribed for the amoeba occur in the Parame- 

 cium. Paramecia defend themselves from 

 enemies, capture and ingest food, digest it, 

 build up protoplasm, react to stimuli, carry 

 on processes of respiration and excretion, 

 and reproduce. 



Offense and defense 



The Paramecium feeds principally on bac- 

 teria and on other protozoans which as a 

 rule are smaller than itself. No special weap- 

 ons of offense appear to be necessary for 

 the capture of food. Paramecia, however, are 

 attacked and used for food by other proto- 

 zoans and by larger animals, so they have 

 a real need for weapons of defense; the 

 trichocysts appear to answer this purpose. 

 These carrot-shaped structures are embedded 

 in the ectoplasm just beneath the surface 

 as shown in Figs. 25 and 26. These bodies 

 are oriented perpendicularly to the surface. 

 A small amount of iodine or acetic acid, 

 when added to a drop of water containing 

 paramecia, causes the discharge of the tri- 

 chocysts to the exterior. After the explosion, 

 the animal is surrounded by a halo of long 



threads. Evidence that the trichocysts are 

 probably weapons of defense is furnished 

 when a paramecium encounters another spe- 

 cies of ciliate, Didinium; see headpiece on 

 first page of this chapter, which shows a 

 Paramecium attached to and being eaten by 

 a didinium. If the seizing organ of this pro- 

 tozoan becomes fastened in the paramecium, 

 a great number of trichocysts are discharged 

 near the place of the injury. If the parame- 

 cium is a large one, it frequently succeeds in 

 making its escape. However, it is probable 

 that the usual function of the trichocvsts is 

 to hold the paramecium to the substratum 

 while it is feeding on bacteria. A ciliate is at 

 a disadvantage during feeding if it does not 

 have anchorage because the ciliaiy currents 

 used to gather food cause it to whirl 

 around. 



Nutrition 



Paramecia do not possess chlorophyl and 

 hence are unable to manufacture food by 

 photosynthesis as euglenas do. One species, 

 Paramecium bursaria, which contains min- 

 ute unicellular green plants in its endoplasm, 

 will reproduce in a solution of salts alone if 

 it is kept in the light. This is a case of mu- 

 tualism in which a mutually beneficial rela- 

 tionship exists between two different or- 

 ganisms. Paramecia do not ingest ever}' small 

 object that reaches the cytostome; they take 

 in certain particles and not others. For ex 

 ample, if different species of bacteria are 

 present, they may feed on one species and 

 not on another. 



Bacteria, yeasts, small protozoans, and 

 algae are captured with the aid of the cilia 

 in the oral groove. By the direction of their 

 beating, they produce a sort of current that 

 drives a steady stream of water toward the 

 cytostome. Food particles that are swept into 

 the cvtostome are carried down into the 

 cytophar}'nx; they are then driven onward 

 by the cilia in the cytopharynx and are fi- 

 nally gathered together near the end into 

 a food vacuole (Fig. 25). When this vacuole 

 has reached a certain size, it is rdc'^scd into 



