I. THE INVERTEBRATES; 



BRANCH PROTOZOA 



(pro to zo'a). 



FIG. 2. 



SINGLE-CELLED ANIMALS. 



This, the introductory Branch, includes those animals 

 which have the simplest structure. The Protozoans are 

 minute forms, seldom equaling a pin's head in size, and 

 are generally found only 

 in salt and fresh water. 

 Their bodies are soft and 

 jelly-like, each contain- 

 ing one or more nuclei, 

 and often presenting a 

 granular mass of food, 

 which has been taken 

 into the central portion of 

 the body. When exam- 

 ined with a microscope of 

 considerable power, there 

 is seen to be a small por- 

 tion, containing a fluid, which is repeatedly sent out in 

 divergent rays through the body substance, or protoplasm. 

 This portion is called the pulsating vacuole (vak'u oi), and 

 serves to force the nutritive products of digestion over 

 the body. 



* The term " Invertebrates " will become obvious further on in the study, 

 when contrasted with the term " Vertebrates." 



t A Diatom (dl'a t&m) is one of the lower plants. 



A mce'ba pro'tl us. A, Indi- B, An Individual 

 vidual that has surrounded a subdividing. The 

 Diatom t (d) from which it is 

 drawing nourishment. Pi; 

 Pulsating or Contractile Vac- 

 uole. N, Nucleus. /, Par- 

 ticles of Food. P, Pseudo- 

 podia. (Greatly enlarged.) 



lower part becom- 

 ing separate from 

 the upper, was ob- 

 served to again 

 subdivide. (Great- 

 ly enlarged.) 



