PHYLUM PROTOZOA 



89 



are dimorphic : the ordinary bell-shaped forms (n.z.) divide in 

 the usual way, but as they remain attached, the process results only 

 in the increased complexity of the colony, not in the development 

 of a new one. The larger zooids (r. z.) are globular and mouthless : 

 they become detached, swim off, and, after a short free existence, 

 settle down, develop a stalk (F), divide, and so form a new colony. 

 In Vorticella multiplication by ItMing also occurs: a small 

 process is given off from one side (Fig. 64, F), develops a basal 

 circlet of cilia, and swims off as a microzooid, the parent individual 



J1U 



FIG. 66. Opalina ranarum. A, living specimen ; B, stained specimen showing nuclei ; C, 

 stages in nuclear division ; D F, stages in fission ; G, final product of fission ; H, encysted 

 form ; I, you^g form liberated from cyst ; K, the same after multiplication of the nucleus 

 has begun ; mi. nucleus. (From Parker's Biology, after Saville Kent and Zeller.) 



or megazooid being left attached to the stalk. Obviously this 

 process is simply a modification of binary fission, the products of 

 division being of very different dimensions, instead of equal-sized as 

 in the more usual case. 



Spore-formation take place in Colpoda. The Infusor becomes 

 encysted, and divides into two, four, and finally eight masses, each 

 of which, becoming surrounded by a special investment, becomes 

 a spore. A somewhat similar process has been described in 

 Vorticella (Fig. 64, IT). 



A peculiar kind of spore-formation, specially adapted to the 

 requirements of an internal parasite, takes place in Opalina 



