CHAPTER I. 



BRANCH L— PEOTOZOA. 



General Characters of Protozoans. — We can imagine no 

 more elementary forms of life than certain members of this 

 branch, whose bodies in the simplest forms a're merely 

 masses of albumen, without any distinct permanent organs, 

 or portions set apart for the performance of any special 

 function. Yet the primary acts of animal life, such as tak- 

 ing food, its digestion and assimilation, and reproduction, 

 are carried on as effectively by these lowest as by the high- 

 est forms. The simplest JProtozoans are like minute drops 

 of protoplasm or albumen, having a gliding motion, and 

 constantly changing their forms, throwing out temporarily 

 root-like projections called 

 pseudopodia, which serve to 

 gather food-particles. Pig. 

 7 illustrates a typical Proto- 

 zoan. It is the common 

 Amoeba of standing water. 

 Most Protozoans are provid- 

 ed with a central organ or 

 nucleus, which corresponds 

 to the reproductive organs of the many-celled animals. 

 The Protozoa are one-celled in distinction from all other 

 animals, from the sponges to man, which are many-celled, 

 though it is claimed that a few shelled forms (Rhizopods) are 

 composed of several indistinct cells. Thus a Protozoan cor- 

 responds to an egg or to any one of the cells composing the 

 bodies of higher animals. They may be naked, a.&inProta- 

 mceba or Amoeba, or may secrete a silicious or calcareous 

 shell. The Infusoria, forming the highest class, are quite 

 complicated, with permanent cilia, a mouth, throat, repro- 



Pig. 7. — Amo9ha, (he nucleus not 6ho\vn. 



