362 GENERAL PRINCIPLES 



symbiosis, and, as examples, we may refer to the green hydra, 

 the green fresh- water sponge and some Protozoa in which cells of 

 a unicellular alga have found the conditions of life favorable 

 within the protoplasm of the animal host. The CO2 eliminated 

 by the animal tissues is food for the plant, while the O eliminated 

 by the plant cells is equally useful to the animal. A similar 

 relation exists between alga^ and fungi of many kinds in the 

 group of organisms called Lichens. Here the algae are wound 

 about by the mycelium of the fungus so that they seem to form 

 a single organism. It has been found, however, that they may 

 be separated and grown independently of each other, and in 

 their structure they show their identity or near relationship 

 with alga? and fungi which are found in nature unconnected. 

 The mutual advantage here is probably like that in the case of 

 hydra, and in addition the alga is protected from the dry air 

 by the dense tissue of the fungus, and the fungus possibly 

 secures soluble food from the alga. 



746. It is difficult to judge of the degree of helpfulness or 

 harm which one organism exercises over another in such a 

 common household. A long list of examples like the following 

 might be enumerated: The little oyster crab which is found 

 at home within the shell of the oyster. A similar crab is found 

 in the tube with certain marine annelids (Cha^topterus). 

 Certain ants capture the cocoons of other ants and rear the 

 young as slaves. Among ant colonies are found a variety 

 of other insects living in more or less harmony, though not 

 always by the consent of the ants. Among the tentacles of 

 certain jelly fish (Cyanaea) a small fish is usually found. 

 Among higher animals a companionship between birds and 

 mammals is often observed. 



747. Parasitism. — More commonly the relationships of this 

 sort are decidedly disadvantageous to the one party. This 

 is then called parasitism, which in several respects is one of the 



