56 ANIMAL PARASITES AND MESSMATES. 



the sexes separate ; and the males, comparatively very 

 small, are attached to the body of each female. It is a 

 case of polyandria which we see realized in the Scal- 

 pellum. Darwin made known the existence of supple- 

 mentary males, so small and so little developed, that 

 they are with difficulty discovered, and so badly are they 

 provided with organs that they have neither those of 

 motion nor a stomach to digest. We have not exhausted 

 the strange peculiarities of this particular group ; there 

 are some which live without shells and claws in the 

 inside of other cirrhipedes, and atrophied males which 

 only exist at the expense of their own females. 



It is almost useless to make the remark that more 

 especially here there exist almost insensible gradations 

 of difference between parasites, messmates, and free 

 animals, and we shall find more than one example of 

 this in the crustaceans to which we now allude. 



The most interesting fixed messmates are evidently 

 those cirrhipedes, which, under the name of Tuhicinella, 

 Diadema, or Coronula, cover the skins of whales. They 

 are, like all the rest, free in their infancy, but soon they 

 take shelter on the back or on the head of one of these 

 huge cetaceans, which they never quit when they have 

 once chosen their abode. That which gives them great 

 importance is, that each whale lodges a particular 

 species ; so that the crustacean messmate is a true flag 

 which indicates in some respect the nationality, and it 

 would not be without interest for voyagers who are 

 naturalists to study these living flags. 



The great whale of the north, the Mysticetus, which 

 our northern neighbours discovered while seeking for an 

 eastern passage to India, a species which never leaves 



