PKOTOZOA 11 



of Paramecium by an opening, called the anus or cytopyge, to 

 the exterior between the mouth and the posterior end. The 

 contractile vacuoles when present, as they are universally in 

 fresh- water examples, are also provided with pores. The 

 ectoplasm bears the cilia. The cilia are free externally to the 

 surface, but pass through the firm cortex to its inner surface, 

 where they end each in a basal grain. The cortex is occupied 

 by a series of minute sacs which open to the exterior. These 

 are termed trichocysts and secrete a thread which can be shot 

 out. In appearance the threads are very like the nematocyst 

 threads of Hydra. ^They are believed to be protective, but in 

 what manner and from what is not clear ; they may be used 

 to prevent the escape of prey. They may prove, in fact, to have 

 a thrombocytic importance, and the thread to be one of mucin. 



The internal surface of the cortical layer is striated by the 

 presence of longitudinal bands, or myonemes. These are 

 contractile and allow of contracting the cell and varying 

 slightly its shape, especially when it is necessary to nego- 

 tiate intricate passages amongst debris. In Paramecium the 

 myonemes have a longitudinally oblique disposition. The 

 superficial layer of cytoplasm in Paramecium, and the fact is 

 generally true of the Ciliata, has been greatly modified to form 

 a protective, sensory, locomotor covering to the cell. 



Eeproduction is regulated to a large extent by tempera- 

 ture. During a continuation of favourable conditions large 

 numbers are formed by transverse fission. Both the nuclei 

 divide and the remaining contractile vacuole is formed in each 

 daughter cell. The macronucleus divides simply or without 

 mitosis, but the micronucleus forms a spindle, and the chromatin 

 gradually appearing is formed into chromosomes, which are 

 divided transversely and migrate to the poles of the spindle. 

 The nucleus is elongated during and after the migration of 

 the chromosomes, and subsequently is constricted and divided. 

 After a number of generations about 170 in Paramecium 

 have been produced in this way it has been found that no 

 further divisions take place until conjugation or syngamy of 

 two individuals occurs. The two individuals fuse by the oral 

 faces. The micronucleus divides by karyokinesis (mitosis) 

 twice. Of the four resulting nuclei one only divides in each 



