GENERAL ZOOLOGY 



the Protozoa should be considered as acellular, or non-cellular, animals — 

 as organisms whose bodies are not equivalent to individual cells but are simply 

 not subdivided into cells. 



Their minute size and soft bodies have made it impossible for most types 

 of protozoans to leave any record as fossils. However, the siliceous skeletons 

 of representatives of the group known as Radiolarians are found abundantly 

 in Pre-Cambrian rock (considerably older than half a billion years) as well 

 as in later deposits. These unicellular animals have thus had an extremely 

 long evolutionary history. 



In this chapter, the Protozoa will be examined as a phylum of the Animal 

 Kingdom and as animals whose unicellular organization may be contrasted 

 with the multicellular organization described for the vertebrates. Also, the 

 capacities of metabolism, responsiveness, and reproduction, which are charac- 

 teristic of all living things, will be examined and compared in protozoans 

 and in vertebrates. 



The Sarcodina 



In the class Sarcodina are included the morphologically simplest forms of 

 Protozoa, although from an evolutionary standpoint the Flagellata as a group 

 are probably the more primitive. A distinctive feature of the Sarcodina is the 

 capacity to form temporary protoplasmic extensions of the body, called 

 pseudopodia ("false feet"). In the subdivision Rhizopoda, which includes 

 creeping forms such as Amoeba, the pseudopodia are lobular or root-like, 

 sometimes subdividing, and may frequently change their shape or be with- 

 drawn. In the Actinopoda, which are floating forms such as Actinophrys, the 

 processes are stiff, rod-like, and more nearly permanent. The name Sarcodina 

 was first applied because protozoans of this class resemble undifferentiated 

 protoplasm, which was originally termed "sarcode," or flesh. Notable among 

 the Sarcodina are the genus Amoeba and related genera, which are collectively 

 spoken of as amoebae or amoebas. 



The Amoeba: General Structure. The protoplasm of an amoeba (Fig. 

 8.1) consists of a thin external layer, the plasmalemma, which functions as a cell 

 membrane; a non-granular region just within, the ectoplasm; and a granular 

 inner region, the endoplasm, in which the nucleus lies. Features of cells 

 like those of a vertebrate are thus apparent. The larger bodies distributed 

 in the cell body, or cytosome, are granules of various sizes, food vacuoles in 

 which digestion occurs, a single contractile vacuole, and other vacuoles con- 

 taining watery fluid and comparable with those found in many other cells. 

 Also present are crystals of definite forms, which may be distinctive for particu- 

 lar species of amoebas; oil globules; and many small inclusions ranging to 

 the limits of microscopic visibility. The significance of these parts will be 

 discussed as necessary in the accounts to follow. 



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