144 ANIMAL BIOLOGY 



called the motor reflex and is performed by Paramoecium 

 in response to all sorts of stimuli, in much the same way. 

 In fact Paramcecium does little else except swim forward 

 and give the motor reflex when it meets with a stimulating 

 agent, so that its behavior is remarkably simple. Some- 

 times it tends to remain quiet with its oral side in contact 

 with some solid object. Advantage is taken of this trait, 

 by placing a bit of cotton wool in a drop of water con- 

 taining Paramcecia, when we wish to keep the creature 

 quiet for study. 



Many infusoria have the property of secreting a coating, 

 or cyst, about themselves, in which they are able to with- 

 stand conditions that would otherwise prove fatal. The 

 cysts of some infusorians have been kept dry for over a 

 year, when they gave rise to living Infusoria after being 

 placed in water. If ponds dry up in summer the infu- 

 sorians, and many other protozoa also, may go into an 

 encysted state until the ponds become filled again. The 

 dried cysts of protozoa may be blown for miles in the dust 

 and thus scatter the species very widely. The great ease 

 with which these minute forms become scattered accounts 

 for the world wide distribution of many species. 



The flagellate protozoa are devoid of cilia but they swim 

 by means of one or more whip-like organs called flagella. 

 Many of them have a mouth by which they take in food 

 (holozoic forms) as the higher animals do; others imbibe 

 food in soluble form through the body wall. Of the latter 

 some live in decaying substances (saprophytic forms), 

 some live within the bodies of other organisms (parasitic 

 forms), and others live by the imbibition of inorganic 

 substances (holophytic forms). One of the latter group, 

 Euglena viridis, combines many characteristics of both 

 plants and animals. This species is provided at the an- 

 terior end with a single flagellum inserted in a small 



