PARASITISM 



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Saprozo/c Animals and Bacterial Feeders. Saprozoic animals may 

 become parasitic if they can withstand the digestive enzymes of the host 

 and the low oxygen tension in its digestive tract. Many of the free- 

 living saprozoic flagellates have parasitic relatives which are specialized 

 so that they can grow only within the digestive tract of particular hosts. 

 Other relatives are intracellular parasites, especially of other pro- 

 tozoa. Other saprozoic parasites such as cestodes and acanthocephalans 

 apparently became saprozoic after they became parasites, for they do 

 not have free-living saprozoic relatives. 



The bacterial feeders that can withstand digestive enzymes and low 

 oxygen tension may become intestinal commensals and feed on the bac- 

 terial population of the large intestine which otherwise becomes a part 

 of the feces. Such commensals are found among flagellates, ciliates, 

 amebas and roundworms. Many of these groups have close relatives that 

 either have become saprozoic and rob the host of digested nutrients or 

 directly attack the host tissues. The most striking case of this kind is 

 found in the ameban genus, Entamoeba (Fig. 39.1). E. coli lives in the 

 large intestine of man and feeds upon bacteria. Although it is abundant 

 in the tropics and by no means rare in temperate regions, it appears to 

 be harmless. It has a close relative, E. histolytica, which also appears 

 to be a bacterial feeder normally but which at times destroys the lining 

 of the large intestine and feeds on red blood corpuscles. An acute at- 

 tack by these parasites can produce severe dysentery and riddle the 

 entire large intestine with deep ulcers and abscesses. 



Parasites may begin as ectoparasites on the host surface or as endo- 

 parasites in the digestive tract. From either of these initial positions 

 the parasites may become endoparasitic among the tissues and organs 

 of the body, or even become intracellular, living within the host cells. 



E. Histolytica. 



Red blood, c&lls 



E.CoH 

 Ba-cteria. 



Cyst 



Cyst 



Fiqure 39 1 A parasite. Entamoeba histolytica (left), and a commensal £ coU 

 (rigMrof the'humrn large intestine. Active amebas above, cysts below that are 

 passed in the feces and can infect new individuals. 



