PHYLUM PROTOZOA. CILIATES 



61 



one end will swim about in all directions, 

 giving the avoiding reaction where stimu- 

 lated, until they become oriented and move 

 toward the cooler end. This is the method of 

 trial and error; that is, the animal tries all 

 directions until one is discovered which al- 

 lows it to escape from the region of injuri- 

 ous stimulation. 



Chemicals of various sorts and in various 

 concentrations have a striking effect on the 

 Paramecium. For example, sodium chloride 

 (salt) solution when allowed to flow under 

 a cover glass (Fig. 28) repels any specimens 

 that encounter it. Weak acetic acid attracts 

 the Paramecium, that is, it reacts positively, 

 and mercuric chloride kills it. 



Frequently a paramecium may be stimu- 

 lated in more than one way at the same 

 time. For example, a specimen which is in 

 contact with a solid is acted upon by tem- 

 perature, chemicals, heat, and other stimuli. 

 The physiologic condition of a paramecium 

 determines the character of its response. 

 This physiologic state is a dynamic condi- 

 tion, changing continually with the processes 

 of metabolism going on within the living 

 substance of the animal. 



Reproduction 



The Paramecium usually multiplies by 

 simple binary fission. This process is inter- 

 rupted occasionally by a temporary union 

 (conjugation) of two individuals and a sub- 

 sequent mutual nuclear fertilization. 



Binary fission 



In binary fission the animal divides trans- 

 versely (Fig. 29). It is an asexual process in 

 which one fully grown individual divides 

 into two daughters without leaving a pa- 

 rental corpse. First the micronucleus under- 

 goes mitosis and its substance is equally 

 divided between the two daughter micronu- 

 clei; these separate and finally come to lie 

 one near each end of the body. The macro- 

 nucleus elongates and then divides trans- 

 versely by amitosis. The cytopharjnx pro- 



duces a bud which develops into another 

 cytopharnj-x. A new contractile vacuole 

 arises near the anterior end of the body, 

 another just back of the middle line. While 

 these events are taking place, a constriction 

 appears near the middle of the longitudinal 

 diameter of the body; this cleavage furrow 

 becomes deeper and deeper until only a 

 slender thread of protoplasm holds the two 

 halves of the body together. This connec- 

 tion is finally severed and the two daughter 

 paramecia are freed from each other. The 

 entire process occupies about two hours. 

 The time, however, varies considerably, de- 

 pending upon the temperature of the water, 

 the quality and quantity of food, and other 

 factors. The daughter paramecia increase 

 rapidly in size, and at the end of 24 hours 

 divide again if the temperature remains at 

 15° to 17° C; if the temperature is raised 

 to 17° to 20° C, two divisions may take 

 place in one day. A paramecium under op- 

 timum conditions may reproduce at the rate 

 of 600 or more generations per year. If all 

 the descendants of one individual were to 

 live and reproduce at a normal rate, they 

 would soon equal the earth in volume. Un- 

 der the usual natural conditions, they do 

 not increase at any such fantastic rate be- 

 cause of lack of food, internal physiology, 

 low temperatures, drought, or falling prey 

 to other animals. 



Conjugation 



Ordinarily the paramecium multiplies by 

 binar}' fission for long periods of time, but at 

 intervals this may be interrupted by the sex- 

 ual process of conjugation (Fig. 30). When 

 two paramecia conjugate, their oral surfaces 

 are opposed, and a protoplasmic bridge is 

 constructed between them. During conjuga- 

 tion the pairs continue to swim about. As 

 soon as this union is effected, the micro- 

 nuclei pass through a series of stages in 

 which the chromosome number is reduced 

 to one-half; this has been likened to the 

 maturation processes of metazoan germ cells, 

 see page 80. These are illustrated and de- 



