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 entosarc. 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 ectosarc 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. 



