298 BIOLOGY: GENERAL AND MEDICAL 



under consideration it is important to start with the idea 

 that the "struggle for existence" is at the bottom of the 

 parasitic association. 



In a general way dissimilar living things are indifferent 

 to one another provided they do not prey upon one 

 another. Sometimes, however, they assume an intimacy 

 so close that where one is found the other can always 

 be expected. Under such circumstances it may justly 

 be surmised that the close association is founded upon 

 some mutual advantage that brings the two together. 



To this communion of life interests the term Symbiosis 

 is applied, and each of the organisms is described as a 

 symbiont. In some cases the symbionts are so closely 

 united as to appear to form a single organism, as in the 

 lichens which consist of an alga upon which a fungus 

 grows. This is described as conjunctive symbiosis. 

 When the symbionts are less closely blended, the relation- 

 ship is described as disjunctive symbiosis. 



Symbiosis may be further subdivided into commen- 

 salism, mutualism, helotism, and parasitism. 



Commensalism. This is a form of symbiosis in which 

 the symbionts derive no known advantage from their 

 intimacy nor do they do one another any harm. Most 

 interesting examples are available. One of the first to 

 suggest itself is the little crab so commonly found in the 

 shell of the oyster. It does the oyster no harm nor 

 does it derive any benefit except that of the defense 

 afforded by the strong shell of its host. When* the shell 

 is open, it is free to enter and exit at will; when it is 

 closed, it becomes a captive. Another perhaps more 

 interesting example is the sea anemone so commonly 

 found upon the shell of the hermit crab. In some of 

 these cases the anemone is attached to the claw of the 

 crab and serves to hide the animal by closing the door 

 of the shell when it retreats. If the crab is able to 

 appreciate this advantage it may explain why, when 

 the animal sheds and is obliged to seek a new home, 

 it seizes the anemone, tears it from its old attachment, 



