PROTOZOA. 27 



by contact with foreign bodies, and therefore acts as an or- 

 gan of touch. The power of active locomotion is enjoyed by 

 most of Protozoa ; but in some cases this is very limited, and 

 in other cases the animal is permanently fixed (as in the 

 sponges). The apparatus of locomotion in the Protozoa is of 

 a varied nature. In many cases, especially in the higher 

 forms, movements are effected by means of little hair-like pro- 

 cesses, which are called " cilia " (Lat. cilium, an eyelash), 

 and which have the power of vibrating or lashing to and fro 

 with great rapidity. In other cases the cilia are accompanied 

 or replaced by one or more long whip-like bristles, which act 

 in the same fashion, and are known as " flagella." Among 

 the lower Protozoa the most characteristic organs of locomo- 

 tion are the so-called " pseudopodia " (Gr. pseudos, falsity ; 

 podes, feet). These consist of variously-shaped filaments, 

 threads, or finger-like processes of sarcode, which the animal 

 can thrust out from any or every part of its body. They are 

 not, however, definite and permanent organs like the cilia, for 

 they can be produced at will, and when they are again with- 

 drawn they simply melt into the sarcode of the body, and 

 leave no traces of their existence. 



As regards the classification of the Protozoa, a rough and 

 useful division is into mouth-bearing or " stomatode " Proto- 

 zoa^ in which there is a distinct mouth ; and mouthless or 

 "astomatous" Protozoa, in which there is no mouth. It is 

 somewhat doubtful, however, if the mouth-bearing forms 

 (namely, the Infusoria) can properly be kept in the Protozoa, 

 so that this arrangement is not a very good one. More scien- 

 tifically, the Protozoa are divided into three great divisions or 

 " classes," known by the names Gregarinidce, Rhizopoda and 

 Infusoria, all of which require special examination. 



CLASS I. GKEGARIOTD^E. The Gregarinidce may be de- 

 fined as parasitic Protozoa which have no mouth, and have no 

 power of giving out pseudopodia. They are usually looked 

 upon as forming the lowest class of the Protozoa / but in all 

 probability much of their degraded character, as we shall see 

 in other cases, is due to the fact that they are internal para- 

 sites, and are therefore not dependent on their own exertions 

 for food. They vary in size from less than the head of a small 

 pin up to nearly half an inch in length, when they look some- 

 thing like small worms ; and they are found inhabiting the in- 

 testines of various animals, especially the cockroach and the 

 earthworm. 



