164 THE PROTOZOA 



Protozoa, however, there is no visible criterion of any similar state 

 of maturity or the opposite which might be a guide in estimating 

 the significance of a particular form. It is certain that with in- 

 creasing knowledge many species of Protozoa now regarded as 

 distinct will prove to be developmental stages of others, as has 

 happened so frequently in the case of Metazoa. 



The polymorphism of the Protozoa may be related directly or 

 indirectly to a variety of causes, which may be grouped generally 

 under three headings life-conditions, growth and development of 

 the individual, and sex. 



1. Polymorphism in Relation to the Conditions of Life. Under 

 this heading are included all those cases where the individual is 

 forced to adapt itself to inevitable changes in the environment, 

 or else succumb to their effects ; hence this type of polymorphism 

 may be termed briefly adaptive. The animal may adapt itself to 

 such changes in one or the other of two ways : passively, by passing 

 into a resting state, in which vital activities are temporarily sus- 

 pended ; or actively, by changes of form, structure, and function, 

 adapted to the changed conditions. 



Methods of passive adaptation to unfavourable conditions occur 

 probably in all Protozoa perhaps it might be said in all Protista, 

 so that no species can be said to be absolutely monomorphic. The 

 commonest form of such adaptation is the process of encystment, 

 whereby the organism protects itself by secreting a firm, resistant" 

 envelope, or cyst, round its body. 



The first preliminary to encystment in Protozoa is usually a 

 rounding off of the body-form. In the case of naked amoeboid 

 forms such a change of form follows naturally, as pointed out 

 above, from cessation of the locomotor activity. It is, however, 

 also observed that a similar change takes place in corticate forms, 

 a phenomenon which indicates that the cuticle or cortex must be 

 absorbed or softened, and that any internal form-giving elements 

 must be dissolved, so that the protoplasm is free to conform to 

 the natural physical tendencies of a fluid body. In the great 

 majority of cases, an individual in process of encystment becomes 

 perfectly spherical, whatever may have been the form of its body 

 in the active state, but in some cases the spherical form is not fully 

 attained, and the body becomes ovoid or pear-shaped. During the 

 process of rounding off, any food-particles or foreign bodies contained 

 in the cytoplasm are rejected or absorbed, as a rule ; the contractile 

 vacuoles, if there be any, cease to be formed and vanish ; and all 

 locomotor organs, such as cilia, flagella, and of course pseudo- 

 podia, are absorbed or cast off. At the same time the protoplasm 

 of the organism becomes less fluid and more opaque, and usually 

 diminishes appreciably in bulk, probably through loss of water ; it 



