230 BIOLOGY 



the little nodules on the roots of plants like peas and beans. 

 If the roots of peas, beans, clover, or similar plants, be care- 

 fully removed from the soil, they will usually be found covered 

 with little nodules ranging in size from the head of a pin to a 

 large pea. These are found to be produced by bacteria which 

 enter the roots and grow and multiply in their tissues. But 

 the association is mutually advantageous. The bacteria are 

 useful in collecting nitrogen from the air which the pea utilizes 

 for its own benefit; and, on the other hand, the bacteria get 

 the benefit of a lodging place and nourishment in the roots 

 of the tubercle, and therefore are themselves benefited by the 

 association. 



Commensalism. In commensalism (Lat. cum = with + 

 mensa = table) the two organisms live together without notice- 

 able advantage or disadvantage to either. As an example, 

 may be mentioned the small crab that lives in the oyster shell, 

 doing no injury to the oyster and gaining no special advantage. 

 Various vines which cling to trees offer another example. 

 Some of these vines force their rootlets into the tissues of the 

 tree and do it injury; these are true parasites. But other 

 vines simply use the tree for the support of their weak, climb- 

 ing stem, and neither plant is particularly benefited or injured 

 by the other, except that the vine is enabled by its climbing 

 habit to send its leaves up into the sunlight. 



Parasitism. In parasitism the mutual relationship is such 

 that one individual is benefited at the expense of the other. 

 The host is always injured, while the parasite is benefited. 

 Among parasites we recognize two types. 



Ectoparasites. Parasites that live upon the outside of their 

 host are ectoparasites. As a rule, they are not very harmful, 

 though they may be so. Among them are some in which a 

 parasitic life is only a part of their existence. The mosquitoes 

 live chiefly upon various juices, but occasionally suck the blood 

 of human beings. In a second class, like the bedbug, the animals 

 live wholly upon the nutrition from their host, but do not 



