I50 PHYLUM PROTOZOA 



They may also leave the body when the irritation is very severe 

 and form a protective halo of bristles surrounding the animal. 



Observe the granular nature of the entosarc, and the spherical 

 food vacuoles within it. These are particles of food, usually com- 

 posed of bacteria or minute particles of other vegetable sub- 

 stances surrounded by water, which circulate within the semifluid 

 entosarc. Watch the entosarc closely, and observe the currents 

 in it. Determine the direction of the currents and whether the 

 direction is ever changed. The food vacuoles form at the inner 

 end of the oral groove, where the particles of which they are com- 

 posed have been swept by the cilia of the groove. Watch the 

 formation of them ; this may be done by putting grains of car- 

 mine or indigo into the water. 



Observe the pulsating vacuoles. These are the excretory organs 

 of the animal. They are globular drops of clear liquid, two in 

 number, which appear near the aboral surface of the body, not 

 far from either end, and break through the ectosarc into the sur- 

 rounding water. They do not appear simultaneously, but alter- 

 nate with each other. When a vacuole has disappeared, radiating 

 canals of clear fluid gradually form about the spot where it was 

 located, bringing the fluid which is to form the next vacuole at 

 that end. Time the formation of the pulsating vacuoles ; how 

 many form in a minute ? 



Observe the macronucleus, a large, ovoid structure near the 

 center of the body. At its side are either one or two minute 

 micronuclei, according to the species, Paramecium caudatum hav- 

 ing one and Paramecium aurelia two ; they may be seen if the ani- 

 mal is killed by adding a i per cent solution of acetic acid to the 

 water or a few drops of aceto-carmine solution. 



Exercise 3. Make a large semidiagrammatic drawing of a Para- 

 mecium showing all these details, and label all. 



Paramecium has no special vegetative organs except the pul- 

 sating vacuoles. Food vacuoles are taken into the entosarc 

 through the mouth. Here they circulate for some time, while 

 the water forming the vacuole is absorbed and the food parti- 

 cles that it contains are digested. The indigestible matters are 



