2o8 THE LIFE OF CRUSTACEA 



and ought to be applied only to cases where two 

 or more animals, living together as " messmates," 

 partake of the same food ; but it is sometimes used 

 more loosely to include instances where one of the 

 animals does not actually share in the food-supply 

 of the other. Parasitism, again, implies that the 

 parasite lives permanently at the expense of its host, 

 by sucking its juices or otherwise, and in this case 

 also there are innumerable degrees and varieties of 

 dependence, which defy inclusion in a strictly logical 

 scheme of classification. Even such typical parasites 

 as Tape-worms, for example, might strictly be regarded 

 as commensals, sharing in the host's food only after 

 it has entered the alimentary canal. Finally, in all 

 these kinds of interrelation, we find cases where 

 the association is temporary, intermittent, or almost 

 accidental, and where there are no perceptible 

 adaptations of structure directed to its maintenance 

 in either of the partners. From these we may trace 

 a series of gradations leading to cases where the 

 associated organisms are never found apart, and 

 where the structure of both is profoundly modified 

 in adaptation to the particular form of association. 



Perhaps the simplest form of association between 

 two animals is found where one utilizes the other 

 as a means of transport. The little Gulf- weed Crab, 

 previously mentioned, is very often found clinging to 

 the carapace or skin of large marine turtles. It is 

 not a parasite, since it can hardly derive any food 



