PHYLUM PROTOZOA 



59 



changes that are injurious or beneficial. (5) Whether a given 

 change shall produce reaction or not often depends on the com- 

 pleteness or incompleteness of the performance of the metabolic 

 processes of the organism under the existing conditions. This 

 makes the behavior fundamentally regulatory." 



Reproduction. - - Paramecium reproduces only by simple 

 lunar \ division. This process is interrupted occasionally by 

 a temporary union (conjugation] of two indi- 

 viduals and a subsequent mutual fertilization. 



BINARY FISSION. - - In binary fission the 

 animal divides transversely (Fig. 39). Both 

 the macronucleus (Fig. 39, N) and micro- 

 nucleus (ri) divide, forming daughter nuclei, oj, 

 A new gullet (V) is budded off from the old 

 gullet (0), and two new contractile vacuoles 

 arise. The animal is then divided into two n ' 

 by a constriction. The entire process occupies ' 

 from about half an hour to two hours. The 

 daughter Paramecia grow rapidly and divide 

 again at the end of twenty-four hours or even 

 sooner, depending on the temperature, food, 

 and other external conditions. It has been 

 estimated that one Paramecium may be re- '"'' dividing by 



binary hssion. N, 

 Sponsible for the production of 268,OOO,OOO jy 1 , "macronucleus; 



offspring in one month. n > n \' mfcronucleus ; 



... o, o l , mouth. The 



. CONJUGATION. - - The conditions that initi- paramcdum figured 

 ate conjugation are not yet known, but the has two mkronudei. 



J (From Sedgwick, after 



complicated stages have been quite fully Hertwig.) 

 worked out. When two Paramecia, which 

 are ready to conjugate, come together, they remain attached to 

 each other because of the adhesive state of the external proto- 

 plasm. The ventral surfaces of the two animals are opposed, 

 and a protoplasmic bridge is constructed between them. As 

 soon as this union is effected, the nuclei pass through a series 

 of stages which have been likened to the maturation processes 



FIG. 39. - - Para- 



