PEOTOZOA 15 



able thus to retreat among the stalked diatoms and other 

 organisms which find a foothold in the same region. When 

 the disturbance has ceased it gradually advances again, the 

 peduncle becoming straight and the bell expanding. 



Should the region become untenable from such causes as 

 lack of food, the bell portion of the cell develops a basal 

 ring of cilia and detaches itself from the peduncle. Thus 

 freed it swims away the disc is the forward end to seek 

 another situation, where it becomes attached and renews its 

 fixed life. It settles down by the base, the basal cilia are 

 withdrawn, and a fresh stalk is developed by an outgrowth of 

 the base of the cell. 



On the other hand, a cyst may be formed, and in this 

 case, as in the others, the body is rounded, the cilia with- 

 drawn, and the ectoplasm in spite of its differentiation is 

 reduced to a relatively simple condition. 



Reproduction is effected in a manner very similar to that 

 of Paramecium. Cell division takes place by the two nuclei 

 dividing and the cytoplasm being split longitudinally from the 

 middle of the disc to the base. One of the daughter cells 

 retains its hold of the peduncle. The other develops a basal 

 ring of cilia and becomes separated. After a period of freedom 

 it settles down and develops a stalk from the basal end of the 

 cell. 



After a number of such divisions have occurred, a change 

 sets in which brings the process to an end. When this last 

 division of the series takes place one of the individuals result- 

 ing undergoes a succession of fissions, thus forming a cluster of 

 small individuals, the microzooids. These are detached and 

 become active swimmers in search of a stalked individual 

 which is in a state demanding conjugation and is able to 

 attract the microzooid. The microgamete, as we may now 

 term the small individual, settles down at the base of the 

 macrogamete and fusion takes place, only the cortical layer 

 of the microgamete being discarded. The micronucleus of 

 the microgamete divides three times and that of the macro- 

 gamete twice, and one of the resulting nuclei in each case 

 divides again. One of the two nuclei resulting fuses with one 

 of the other, all the rest disappearing. The nucleus derived 



