CHAPTER VII. 



COMMENSALISM AND PARASITISM. 



THE term Symbiosis has been applied by natur- 

 alists to the phenomenon of the living together 

 for mutual help or protection of different species 

 of animals or plants. It is a well-known fact, 

 to all those who have taken an interest in any 

 large group of animals, that some species are 

 nearly always associated with other species, be- 

 longing perhaps to a different class altogether, 

 and very frequently mimicking them in form or 

 colour. At first it might be thought that most 

 of these cases could be dismissed as cases of para- 

 sitism ; but when the careful observer notices that 

 neither of the species is injured by the associa- 

 tion, the conditions of the partnership are evi- 

 dently very different to those of a blood-sucking 

 parasite and its ungracious host. 



Besides the words Symbiosis and Parasitism, 

 the terms Commensalism and Mutualism have 

 been applied to various cases of association of 

 different species of animals ; but with the increase 

 of our knowledge of the habits of animals, it is 

 becoming more and more difficult to classify 

 all known cases under these four heads, and the 

 words are consequently often used with widely 

 different meanings. 



It will be perhaps the best plan to adopt in 

 this book, to avoid any attempt to give defini- 

 tions of these terms until a few cases illustrative 

 of each have been described. 



One of the commonest objects of the sea-shore 



146 



