SUBKINGDOM 



PROTOZOA 



This division, created by Yon Siebold in 1845, has been gener- 

 ally adopted by naturalists as a convenient receptacle for those 

 lowest minute forms of animal life which do not readily fall into 

 any one of the other subkingdoms. From their extreme sim- 

 plicity the Protozoa possess few positive characters. They are 

 unicellular, and consist of slightly modified protoplasm called 

 " sarcode." In the lowest forms (Monera) even the nucleus of 

 the cell is wanting, and the animal is termed a cytode. 



In the typical Protozoa, the outer layer of the cell is denser 

 and more homogeneous than the internal protoplasm, which is 

 filled with dark particles called "granules," and clear spaces 

 termed " vacuoles." Many of the latter are only temporary, but 

 usuall}^ one is larger than the rest, permanent and fixed, and 

 dilates and contracts at regular intervals. It is termed the "con- 

 tractile vesicle." There may be more than one contractile vesi- 

 cle, and more than one nucleus. No tissues are present. The 

 highest oi'ganization is seen in the Infusoria, which possess merely 

 a rudimental food-cavity, and permanent cilia. The nucleus and 

 contractile vesicle seem to be rudimental organs of reproduction 

 aud circulation. Notwithstanding the lack of organs and struc- 

 tures possessed by the higher animals, the Protozoa carry on all 

 the primary animal functions. 



The Protozoa may be divided into the classes Infusoria, Gre- 

 garinida, Rhizopoda and Monera. The Monera are sometimes 

 placed with the Phizopods, near the Amoeba. They, however, 

 are simpler than the Amosba, as they possess no nucleus or con- 

 tractile vesicle. Only the Phizopods occur as fossils. 



