56 



NATURAL HISTORY 



a shark- 



s-cccker- 



protozoans receive food and are protected by their hosts. A some- 

 what similar situation prevails in the large intestine of man, where 

 certain types of useful bacteria are found. These forms help keep 

 down putrefying bacteria, receiving in return a home, food, and 

 a favorable temperature in which to live. Certain species of ants 

 protect and feed aphids, in turn feeding upon the sweet fluid secreted 

 by the aphid. 



Commensalism 



Some associations are not obviously to the advantage of either 

 organism, the two feeding together as messmates. Animals like 



the small crabs that live 

 in the water canals of 

 certain sponges, or the 

 tiny fishes that live in 

 the lower part of the 

 body of a "trepang," a 

 sea cucumber, are ex- 

 amples. The young of 

 some species of rudder- 

 fish (Stromateidae) ac- 

 The shark sucker (Remora brachypiera, Lowe) company large jellyfish, 

 showing sucking disk and its method of attach- geekina; shelter under 

 ment to the shark. The Remora gets free trans- ,, ■ +• • j. 4. i 



portation and makes sudden forays after food as their stmgmg tentacles 

 well as sharing the "left-overs" of the shark's food, when chased by larger 

 But it seems doubtful if the shark gains anything ggj^ while another fish 

 from the association. . ^ . 



{Nomeus) lives in con- 

 stant association with the beautiful coelenterate known as the 

 Portuguese man-of-war. 



Parasitism 



Not all life is give and take. Some plants and animals live at the 

 expense of others, giving nothing and taking all. These are known 

 as parasites, the organism which entertains them being called the 

 host. From the lowest to the highest forms in the plant and animal 

 kingdom there are few which are not attacked by parasites at some 

 stage of their existence. 



Parasitism implies plenty of food, shelter, and a relatively protected 

 life for the parasite, but it also usually spells degradation in structure 

 and loss of activity. It may mean only inconvenience, but more 

 Ukely a shorter and disturbed life for the host, especially if the parasite 



