INVERTEBRATE ANIMALS, 



SUB-KINGDOM L PROTOZOA. 

 CHAPTER I. 



i. GENERAL CHARACTERS OF THE PROTOZOA. 2. CLASSIFICA- 

 TION. 3. GREGARINID^E. 



THE sub-kingdom Protozoa (Gr. protos, first ; and zodn, 

 an animal), as the name implies, is the lowest division of the 

 animal kingdom, and its limits are therefore necessarily not 

 yet strictly defined. The Protozoa comprise an enormous 

 number of animals, almost all of which are so small as to be 

 invisible to the naked eye, and can only be satisfactorily ex- 

 amined under pretty high powers of the microscope. For 

 this reason, and because they are almost universally found in 

 water, these creatures, often popularly called " animalcules," 

 are almost unknown to the majority of people. Some few, 

 however, attain a large size, and of these the sponges are 

 familiar examples. The microscopical forms of the Protozoa 

 swarm in most stagnant pools, and in all waters charged with 

 organic matter so as to afford them food. Every worker with 

 the microscope is familiarly acquainted with them, and they 

 exhibit phenomena which in many cases render them objects 

 of the highest interest. From their low position in the ani- 

 mal scale, it arises that the Protozoa are mainly characterized 

 by the absence of organs and structures which occur in higher 

 beings, and they possess few positive characters by which they 

 can be distinguished. 



The Protozoa may be defined as animals, generally of 

 very minute size, composed of a nearly structureless, jelly-like 



