COMMENSALISM AND PARASITISM. 157 



water is considerably disturbed by these currents, 

 and the constant and rapid flow of fresh water 

 bearing food-organisms benefits, not only the 

 Barnacles, but also the Millepore polyps in their 

 neighbourhood. 



Thus the Barnacles may be a benefit to the 

 Millepores in which they live. It cannot be 

 asserted, however, that this probability is a 

 proved fact. A great deal more knowledge 

 about the rate of growth of the Corals which 

 are and are not affected, must be acquired 

 before such an assertion could be made : but the 

 probability that the Barnacles may be of service is 

 sufficient to cause us to hesitate before branding 

 them with the epithet of " parasites." 



This particular case, which has been given 

 above in some detail, may be regarded, in a 

 sense, as a test case, because other animals 

 besides the Barnacles, which gain their food by 

 producing currents, are found in Corals. Such 

 are the tubicolous Worms, bivalve Molluscs, and 

 certain Sponges. So plentiful are these on the 

 older branches of some Corals, that quite a rich 

 Fauna belonging to several groups of animals 

 may be found by carefully studying them. 

 These might all be dismissed as parasites by 

 the non-inquisitive mind, but many of them, at 

 any rate, may be regarded by the more cautious 

 naturalist as not injurious, and others perhaps as 

 positively beneficial to the Coral on which they 

 live. There is a very curious case of symbiosis 

 mentioned by Semper, which may be related 

 here as similar in some respects to those above 

 quoted. 



