186 INVERTEBRATE ZOOLOGY 
as it goes. At its inner end the groove becomes a closed 
tube, which extends into the entosarc and ends with the mouth. 
Notice the trichocysts — slender, radially arranged bodies which 
fill the ectosarc. They are organs of defense, which remind 
one of the nematocysts of the Cnidaria; when the animal is 
irritated they discharge long, delicate bristles, which project 
beyond the cilia or may leave the body. 
Observe the granular nature of the entosarc, and the spheri- 
cal food vacuoles within it. These are particles of food, usually 
composed of vegetable substances surrounded by water, which 
circulate within the semifluid entosare. Watch the entosarc 
closely, and observe the currents in it. Determine the direc- 
tion 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 composed have been swept 
by the cilia of the groove. Watch the formation of them. 
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 ectosare into the 
surrounding water. They do not appear simultaneously, but 
alternate 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, P. caudatum having one and 
P. aurelia two; they may be seen if the animal be killed by 
adding a 1 per cent. solution of acetic acid to the water. 
Exercise 3. Make a large semidiagrammatic drawing of a Para- 
mecium, showing all these details, and label all. 
