PARASITIC ADAPTATION. 95 



the association is forced, though not benefitting, may in no 

 way suffer from it. A familiar form of this living together 

 (commensalism) is the little crab so commonly found in the 

 shell of the oyster. The oyster is not harmed by its presence, 

 but the crab is benefitted by the protection which the shell 

 affords. Another more curious example of such association 

 is afforded among the vertebrates by the species of Re^tiora, 

 or suck fishes, which have the first dorsal fin modified into a 

 sucking disc on top of the head. By means of this disc it 

 attaches itself to a shark or other large fish, and is thus 

 carried about, detaching itself only to secure food. Its bene- 

 fit from such association is in being carried to new feeding 

 grounds without effort of its own, and in the shelter from its 

 enemies which the body of the larger fish may afford. The 

 host, on the other hand, cannot be benefitted, nor does it 

 seem to suffer by the presence of its uninvited guest. 



Whether this relationship between different species is of 

 reciprocal advantage, or of benefit to but one, neither of the 

 symbionts lives upon or at the expense of its co-symbiont, 

 and neither has entirely renounced its independence. In true 

 parasitism the invading animal lives at the expense of its 

 host. We have many familiar examples of this form of sym- 

 biosis, and the conditions that seem always to attend it, such 

 as the degeneration, slight or extreme, of the parasite, are 

 familiar to all observers of animal life. It is the common 

 habit of many animals, however, to prey upon the bodies of 

 other animals, and we should distinguish, so far as we may, 

 between those which are predatory in habit and those which 

 are parasitic. The former are free, and exercise their powers 

 of sense and cunning in snaring or chasing their prey, while 

 the latter live on or in the bodies of their victims, often bur- 

 rowing into and consuming the body substance, leading a 

 lazy, beggarly existence in which all the faculties of special 

 sense and prowess, so highly developed in predatory animals, 

 become degenerate through the atrophy of disuse. 



Parasitism is found throughout the range of animal life 



