PHYLUM PROTOZOA 77 



off and passes into the endoplasm as a food vacuole. Observe in what part of 

 the body the food vacuoles first collect and where they are found later. Find 

 out by observations at intervals their exact course in the endoplasm and make 

 a diagram to indicate your observations. The movement of the vacuoles is of 

 course due to a slow circulation of the endoplasm. You may, if you watch the 

 preparation long enough, be able to observe the discharge of the carbon through 

 the anus. 



b) Discharge of the trichocysts: Mount some Paramecia on a slide, add a 

 drop of picro-acetic acid, cover gently, and examine. Each animal will be found 

 surrounded by a halo of long threads which are the discharged solidified contents 

 of the trichocysts. Make a drawing showing relative lengths of cilia and the 

 threads from the trichocysts. 



c) The avoiding reaction: Mount some Paramecia as usual and observe with 

 the low power. What happens when the animal strikes an obstacle? This 

 reaction is called the avoiding reaction. Make a diagram to illustrate the reaction, 

 showing the position of the Paramecium before, during, and after striking the 

 object. 



d) Reaction to chemicals: Obtain a considerable number of Paramecia and 

 spread them out over the slide. Place the slide on the table and drop a small 

 crystal of common salt in the center of the slide. Observe with the naked eye 

 the behavior of the Paramecia toward the salt. What does each Paramecium 

 do on coming into the neighborhood of the salt solution? What part does the 

 avoiding reaction play in the behavior? Make a diagram to illustrate the results 

 of this experiment. 



4. Reproduction. This process takes place by division (fission) of the animal 

 into two halves by a transverse constriction. A sort of sexual act called con- 

 jugation also occurs at intervals, although it has been demonstrated that this 

 is not necessary for the continued existence of Paramecium (Hegner, p. 73). 



a) Fission: In fission, both of the nuclei play an active role. The mega- 

 nucleus divides by direct division; the micronucleus by a very primitive kind 

 of mitosis, with the formation within it of fibers like the spindle fibers (see 

 Hegner, Fig. 32, p. 70). Study and draw from the prepared slides three stages 

 of fission, marked "early," "middle," and "late." Note that each slide contains 

 only a few individuals which are dividing while the rest of the specimens are 

 merely normal. The location of the dividing individuals has been indicated on 

 most of the slides by ink marks. Do not draw until you are sure that you have 

 found a specimen in fission. 



(i) Early fission: The beginning of fission is recognizable by the elongation 

 of the meganucleus, which becomes nearly as long as the animal. The micro- 

 nucleus has moved away from its usual position in a depression of the mega- 

 nucleus and is free in the cytoplasm, where it will be found in various stages of 

 division. It accomplishes this division by pulling into two halves which move 



