112 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



group. In many of the Hsemosporidia a part of the life cycle takes 

 place in an intermediate host, usually a mosquito or a tick. 



Other groups of the Sporozoa are the Myxosporidia, with peculiar 

 nematocyst-like organs (Invertebrates and cold-blooded Vertebrates), 

 and the Sarcosporidia, which are found inside the striped muscles of 

 warm-blooded Vertebrates. 



GENERAL NOTES ON THE FUNCTIONS OF PROTOZOA 



Movement. The simplest form of movement is that 

 termed amoeboid, as illustrated by an Amoeba. In ordinary 

 conditions it is continually changing its shape, putting forth 

 blunt lobes and drawing others in. With this is usually 

 associated a streaming movement of the granules. A more 

 defined contraction, like that of a muscle cell, is illustrated 

 in the contractile filament of the stalk of Vorticella and similar 

 Infusorians ; and not less definite are the movements of cilia 

 and flagella, by means of which most Infusorians travel 

 swiftly through the water. Cilia in movement are bent and 

 straightened alternately; while flagella, which are usually 

 single mobile threads, exhibit lashing movements to and fro, 

 or, more often, are held stretched out in front, and by a 

 curious rotatory movement draw the cell along. They are 

 then more aptly termed tractella. It seems probable that 

 cilia and flagella consist of an elastic core surrounded by 

 a sheath, which may be uniformly contractile, or may have 

 one contractile band, or two opposite contractile bands, and 

 so on. 



Considered generally, the movements are of two kinds: either (i) re- 

 flex, i.e. responses to external stimulus, as when the Protozoon moves 

 towards a nutritive substance ; or (2) automatic, i.e. such movements as 

 appear to originate from within, without our being able to point to, the 

 immediate stimulus, e.g. the rhythmical pulsations of contractile 

 vacuoles. Actively moving Protozoa usually show the following motor 

 reaction to stimulus : they move backward, turn over on one side 

 structurally defined, and then move forward again. 



Sensitiveness. The Amoeba is sensitive to external influ- 

 ences. It shrinks from strong light and obnoxious materials ; 

 it moves towards nutritive substances. This sensitiveness 

 is, so far as we know, diffuse a property of the whole of 

 the cell substance ; but the pigment spots of some forms 

 are specialised regions. 



