CHAPTER LXV 



ASSOCIATION OF ANIMALS MESSMATES OR 

 COMMENSALS (COMMENSALISM) 



Messmates are organisms of different species which are more 

 or less closely associated, to the benefit of at least one partner in 

 the concent Cases of this sort are grouped under the head of 

 Commensalism Mutualism (Symbiosis] is a much more intimate 

 kind of relation between two organisms, to the advantage of both, 

 as already described. The best examples of such Mutualism or 

 Symbiosis involve a partnership for certain cases where plants 

 and animals are thus associated (see p. 75). It is doubtful 

 whether any two kinds of animal live together in this intimate 

 fashion. Parasites are animals which live on or in other animals, 

 at their expense, and to their detriment. Parasitism also includes 

 cases where one organism concerned is a plant (see p. 76). 



In a broad sense all the animals which live and feed together 

 in the same place may be regarded as messmates, and the rela- 

 tions between such species may be very complex. It will, how- 

 ever, be well to restrict the term to cases where the connection 

 is of closer and more constant nature, involving the interests of 

 definite species. But it must not be forgotten that this kind of 

 association has no doubt gradually arisen from relations which 

 were originally of more casual kind. So many instances of Com- 

 mensalism are known that it will only be possible to describe a 

 few of the more striking examples. 



FISHES (PISCES) AS MESSMATES 



Some extraordinary cases have been described where small 

 bony fishes take up their quarters within the digestive organs 

 of lower animals, sallying forth from these peculiar refuges as 

 circumstances dictate. The most familiar instance of this is 



170 



