198 ASSOCIATION OF ORGANISMS 



Parasitism. — It is a very short step indeed from mess- 

 matism to parasitism. Let the crab, living as a mess- 

 mate in the sponge, begin to feed upon the substance of 

 the sponge and it becomes a parasite. Parasitism, there- 

 fore, is an association, with one member, the parasite, 

 gaining its livelihood from the other, termed the host. 

 Associations of this nature are exceedingly common. 

 With animals, at least, it has been affirmed that every 

 species, save the very smallest, serves as host for at least 

 one species of parasite. 



Many parasites, such as ticks, fleas, and leeches, live 

 on the outer surface of the host, and are termed external 

 parasites. Others living in the digestive tract, muscles, 

 or blood stream are called internal parasites. Also, there 

 are many species, such as the tapeworm and trichina, 

 whose organization requires that they spend their entire 

 existence in one or at most two or three hosts. These 

 are permanent parasites. Finally, there are many others, 

 such as the flea and mosquito, that move freely from one 

 host to another and are classed as temporary parasites. 



Parasitic Plants. — There are also many plants that 

 are parasitic upon other plants or animals. Among the 

 higher types the best known example is probably the 

 mistletoe, species of which attack forest and culti- 

 vated trees. It is a bushy shrub (Fig. 48) with yellow- 

 ish green leaves, and is attached to the host by means of 

 specially modified roots which are believed merely to ab- 

 sorb water and dissolved salts. These substances are 

 then combined into more complex compounds by the 

 activity of the parasite itself. 



In the dodders, a group of plants related to the morn- 

 ing glory, and attacking many kinds of herbs and low 

 shrubs, the leaves are absent and the stem appears like 

 a twining yellow or red thread (Fig. 49). At frequent 

 intervals the stems and branches are provided with roots 

 which absorb from the host plant the water, sugar, and 

 mineral salts necessary for life of the parasite. When 



