The kind of nutrition practiced by a para- 

 site depends largely upon its evolutionary 

 background. Many parasitic animals, for ex- 

 ample, are essentially holozoic, in the sense 

 that they ingest the blood or tissues of their 

 hosts. But other parasites get their nourish- 

 ment essentially like saprophytes, absorbing 

 organic substances directly from the body 

 fluids, tissues, or cells of their living hosts. 



Many parasites do little or no harm to the 

 hosts, aside from "stealing" a certain quan- 

 tity of organic material. But many produce 

 definite diseases in the hosts, either by direct 

 injury to the cells and organs, or by produc- 

 ing toxic waste products. Among animal 

 diseases, especially in man, many are caused 

 by bacterial parasites (for example, diphthe- 

 ria, pneumonia, tuberculosis, gonorrhea); 

 others are caused by parasitic protozoans (for 

 example, malaria, African sleeping sickness, 

 syphilis); others by viruses (infantile paral- 

 ysis, smallpox, mumps, etc.); and others by 

 multicellular parasites (hookworm, trichino- 

 sis, etc.). Among plant diseases, most are 

 caused by parasitic fungi (rusts, smuts, molds, 

 mildews, etc.), although a few are caused by 

 viruses. 



Symbiosis. In certain cases not only the 

 parasite but also the host derives benefit 

 from their association. Such a mutually bene- 

 ficial relation between parasite and host is 

 called symbiosis. In this category there are 

 many interesting cases. The lichens, for ex- 

 ample, are essentially dual organisms, for 

 among the cells of these multicellular fungi 

 there are always a number of unicellular 

 green algae (Fig. 10-5). The alga donates its 

 extra carbohydrate to the fungus, and the 

 fungus aids the alga by contributing water 

 and salts, absorbed in excess through the 

 hyphae. In this case the symbiotic organisms 

 have become completely dependent upon 

 each other, and neither is able to live very 

 long in the absence of the other. 



Some fungi live symbiotically on and in 

 the roots of higher plants (for example, 

 trees). These fungi aid the host plants in 

 absorbing water and salts, through part of 



Other Modes of Nutrition; Conservation of Food Elements 



FREE SURFACE. 



179 



ALGAE 

 FUNGUS 



SUBSTRATUM 



Fig. 10-5. The lichen is a dual organism, which pro- 

 vides a good example of symbiosis. Section of the 

 mycelium showing the cells of a green alga nestled 

 among the colorless hyphae of the fungus. 



the mycelium that ramifies out among the 

 soil particles; in return, the root of the host 

 provides the fungus with the preformed 

 carbohydrates. Many herbivorous animals, 

 such as the cow, develop rich cultures of 

 cellulose bacteria in the digestive tract. Here 

 the bacteria convert considerable quantities 

 of cellulose into glucose that is absorbed in 

 part by the host and in part by the parasites. 

 In return, the bacteria are afforded ideal con- 

 ditions of warmth and darkness while they 

 absorb their other essential nutrients (water, 

 salts, etc.) from the digesting food mass as it 

 passes through the alimentary canal of the 

 contented host. 



Neutral Parasites. In some cases, a para- 

 site neither harms nor benefits the host in 

 any tangible fashion. Such neutral parasites 

 are, perhaps, even commoner than the harm- 

 ful and beneficial types. For example, a num- 

 ber of species of bacteria inhabit the human 

 digestive tract, especially in the region of the 

 large intestine (p. 314). Aside from the fact 

 that these bacteria appropriate small quan- 

 tities of organic foodstuffs, no definite ill 

 results from their presence in the gut. In 

 fact, some of the intestinal bacteria of man 

 are now known to benefit their host, by syn- 

 thesizing several of the B vitamins, of which 

 some may be absorbed by the host. 



