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 concern. Cases of this sort are grouped under the head of 
Commensalism. Mutualism (Sydzoszs) 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 
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