2 PROTOZOA. 



In a large number of the Frotozot true sex- 

 ual reproduction has not yet been proved to take 

 place. 



In habit the Protozoa are almost exclusively 

 aquatic. Some attain to an appreciable, and even 

 (in the case of the Sponges) considerable, size; but 

 by far the greater number are minute, and hence, 

 notwithstanding their abundance, the ignorance 

 which still prevails as to their real nature. 



2. Classiiieatlon. — The following animal 

 groups are included in this sub-kingdom, viz. : — 



J. Rhizopoda, 



2. Polycystina, 



3. Spongidcey 



4. ThalassicollidcB, 



5. Gregarinidce, 



6. Infusoria, 



The presence of a mouth is characteristic of the 

 Infusoria, and hence the remaining Protozoa are 

 sometimes designated by the collective appellation 

 of ' Astomata.^ 



