CHAPTER 9 



PROTOZOA 



There is nothing funny in the thought that even man, who 

 was made in the image of God, bears about in his vital organs 

 various forms of loathsome creatures, which riot on his fluids 

 and consume the very substance of his tissues. 



Philip Henry Gosse 



ANIMALS which perform all the functions of life within the compass of 

 a single cell outnumber all the other animals by a million to one. 

 These single-celled organisms, which are known as Protozoa (Fig. 2), 

 vary considerably in size but the largest are only just visible to the 

 naked eye. The simplest forms like amoeba consist of a blob of proto- 

 plasm containing a nucleus. In a fluid medium they sometimes 

 assume a spherical form and under the microscope each is somewhat 

 reminiscent of a fried egg — although the nucleus is colourless, not 

 yellow like the egg yolk. In some of the parasitic forms, such as the 

 Coccidia, the body has a spherical or ovoid shape which lies motionless 

 within the cytoplasm of the host's cells. On the other hand many types 

 which live in lymph or blood and other body fluids vary considerably 

 in appearance and structure. They are endowed with the power of 

 active movement like the free-living Protozoa which swarm in water 

 and damp situations. 



It is generally believed that the parasitic forms are derived from free- 

 living ancestors, and as almost every higher animal harbours one or 

 more species of parasitic, commensal or symbiotic Protozoa, the number 

 of dependent forms is large. Although only single-celled organisms, 

 they display many of the adaptations to the parasitic mode of life which 

 are found in multi-cellular animals. Thus, in some forms special organs 

 of attachment are developed. A good example of this type of structure 

 is found in the sucking disc o^Giardia (Fig. 2, g & h) — flagellate which 



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