PROTOZOA. 



49 



of a higher grade. A nervous system is universally and en- 

 tirely absent, and the sole circulatory apparatus consists in 

 certain clear spaces called "contractile vesicles," which are 

 found in some species, and which doubtfully perform the 

 functions of a heart. A distinct alimentary aperture is present 

 in the higher Protozoa, but in many there is none ; and in all, 

 the digestive apparatus is of the simplest character. Organs 

 of generation, or at any rate differentiated portions of the 

 body which act as these, are sometimes present ; but in many 

 cases true sexual reproduction has not hitherto been shown to 

 exist. 



The ^sarcode," which forms such a distinctive feature in 

 all the Protozoa, is a structureless albuminous substance, not 

 possessing " permanent distinction or separation of parts/' but 

 nevertheless displaying all " the essential properties and char- 

 acters of vitality," being capable of assimilation and excretion, 

 of irritability and of the power of contraction, so as to produce 

 movements, strictly analogous, in many cases, to the muscular 

 movements of the higher animals. In some, too, the sarcode 

 possesses the power of producing an external case or envelope, 

 usually of carbonate of lime or flint, and often of a very com- 

 plicated and mathematically regular structure. 



The power of active locomotion is enjoyed by a great many 

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

 in other cases the animal is permanently fixed in its adult 

 condition. The apparatus of locomotion in the Protozoa is of 

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

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

 processes which are known as "cilia," and which have the 

 power of lashing to and fro or vibrating 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." The most characteristic organs 

 of locomotion amongst the lower Protozoa are known as 

 " pseudopodia," and consist simply of prolongations of the 

 sarcodic substance of the body, which can usually be emitted 

 from the greater portion of the general surface of the body, 

 and are capable of being again retracted, and of fusing com- 

 pletely with the body-substance. 



2. Classification of the Protozoa. The sub-kingdom Protozoa 

 is divided into three classes viz., the Gregarinida, the Rhizo- 

 poda, and the Infusoria. In the Infusoria only 4s^ a mouth 

 present, and hence these are sometimes spoken of as the 

 " Stomatode" Protozoa, whilst the two former classes collec- 

 tively constitute the " Astomata.?. 



ft 



