30 The Under-Water World 



of its host for the purposes of incubation, 

 whilst drawing its sustenance from some 

 outside source. The Crustacea offer 

 numerous examples of this form of 

 parasitism. One small form attaches 

 itself to the tongues of flying-fish. 

 Another lives and hatches its eggs in the 

 gill cavity of a prawn, causing a pro- 

 nounced swelling, and earning for its 

 host the fisherman's ribald name of 

 " face-ache " prawn. Yet another crus- 

 tacean Sacculina found attached to 

 the abdomens of crabs is little more than 

 a sackful of eggs, which hatch into 

 free-swimming creatures suggestive of 

 young barnacles. Such parasites are so 

 degenerate that when adult they pos- 

 sess no locomotory organs, showing no 

 indication of the class of animals to 

 which they belong, and their true position 

 in the animal kingdom would have re- 

 mained unsolved were it not for the clue 

 presented by their newly-born offspring. 



