FIFTH SUB-KINGDOM. 



PROTOZOA. 



The division of Protozoa, created by Von Siebold in 1845, has been 

 extensively adopted by naturalists as a convenient receptacle for many 

 forms of animal life which do not fall into any one of the four great 

 Sub-kingdoms established by Cuvier. Their bodies, composed of a 

 peculiar semi-gelatinous substance called "sarcode," possess neither mus- 

 cular nor nervous tissue, and the circulatory and digestive systems are 

 most feebly defined. A large proportion of these organisms consist 

 simply of cells, single or agglomerated. 



Naturalists entertain many and widely diverse views as to the legiti- 

 macy of the group, or its value, if allowed. Agassiz does not recognize 

 the Protoza as a distinct branch of the Animal Kingdom ; while Owen 

 takes the bold step of creating for their reception a third primary divi- 

 sion of the organic world, intermediate between animals and plants. 

 With the majority of comparative zoologists, however, the group is 

 gaining ground, although much has yet to be done towards the attain- 

 ment of a perfect knowledge of their structure and development before 

 their limits can be well defined, or a good classification can be attempted. 

 The three more commonly recognized Classes are : Infusoria, Rhizopoda 

 and Porifera. 



CLASS L— INFUSORIA. 



These animalcules were first observed by Loewenhdck in 1675 ; 

 and our present knowledge is chiefly due to the labors of Ehrenberg, 

 Pritchard, Dujardin and Bailey. The later researches have tended to 



