COMMENSALISM AND PARASITISM. 159 



It seems very improbable that these cage-like 

 swellings upon" the branch can be of any great 

 disadvantage to the Coral. It is true that they 

 destroy the beautiful symmetry of the branch, 

 and give it a distorted and diseased appearance ; 

 but this is only an aesthetic disadvantage, which 

 does not probably count for much in the struggle 

 for existence on the Coral reef. To the Crab the 

 arrangement is undoubtedly an advantage, as it 

 gives it a secure position, free from the attack of 

 its ordinary foes, where food is probably abun- 

 dant and easily obtained. 



The skin of Whales is often beset with Bar- 

 nacles ; in fact some species of them are found 

 nowhere else but on these Mammals. They are 

 usually deeply embedded in the skin, only a 

 small round hole through which the legs can 

 be protruded, communicating with the exterior. 

 These Barnacles do not feed upon the tissues and 

 juices of the Whale, but, in the usual manner of 

 the non-parasitic Barnacles, upon organisms that 

 swim freely in the water. The advantage to the 

 Barnacles is obvious, as the movements of the 

 Whale through the water must bring them in 

 reach of constant fresh supplies of food, but the 

 benefit to the Whale is not so clear. It cannot 

 be supposed for a moment that the Barnacles 

 assist the Whales in their search for food, nor can 

 they be regarded, when present in great numbers, 

 as a protection to the skin by the strength 

 afforded by their thick calcareous shells ; at the 

 same time there is no reason to suppose that 

 their presence is an inconvenience or in any way 

 harmful to the Whales. 



