86 



Protozoa. 



ofE (Infusoria) : or the outer layer may constitute a definite and 

 well-defined, althougL. flexible, coat, as in the ectoplasm of Grega- 

 rines. In both cases the body is, however, usually capable of 

 considerable change of shape. Where the form is fairly constant,, 

 the body is often beset with cilia or flagella, varying in size and 

 number. Almost all the Protozoa are microscopic in size. 



Reproduction, like multiplication in all other cells, occurs 

 by fission; the nucleus dividing first and then the protoplasm. 

 Usually the fission is binary, but very often the animal breaks up 

 simultaneously into a large number of small individuals (spores). 

 Sometimes division occurs by a process of gemmation, a small 

 portion being constricted off to form a new individual. 



A great many forms, of various divisions, have the power 

 of encysting if the conditions are unfavourable, emerging 

 again when they improve. The body becomes spherical and 

 secretes a membrane of varying thickness over its whole surface, 

 and in this state it is able to survive complete desiccation : thus 

 encystation occurs if the water in which the ^organism is living 

 begins to dry up : or, again, if it is too stagnant, or when the 

 food is' exhausted, or, after the animal has ingested a large supply 

 of food, when it remains at rest until digestion is completed. 



.;.....y 



Fig. 45. 



Fig 45. An Infuaorian in the free state (left), and encysted (right). N nucleus (macro- 

 nucleus). — ^After Balbiani. 



Fig. 46. An encysted Infnsorian which has broken up into a number of spores, now 

 in the act of leaving the cyst. — After Fourquet. 



Lastly, it may encyst before undergoing fission. The 

 protoplasm then divides into spores (frequently a very large 

 number), which later on leave the cyst (c/., the Gregarines ; the 

 same thing also occurs in other forms, e.g., in the Infusoria). 



In many Protozoa, conjugation may sometimes be observed : two 

 individuals of the same species approach one another and fuse, a 

 fusion of the nuclei also occurring. This conjugation recalls the 

 fertilisation of the Metazoa, which also consists in a fusion 

 of two cells and their nuclei. Very often it is followed by repestted ■ 



