THE WEB OF LIFE 289 



of the cases are difficult. Thus Dr. W. T. Cahnan describes 

 a crab from Christmas Island which had a hydroid polyp, 

 allied to Stylactis, attached like a tassel at the ' knee ' 

 of each of its legs. All but two of the polyps were symme- 

 trically disposed and the rootwork (or hydrorhiza) followed 

 the grooves on the carapace. Moreover, the type speci- 

 mens of the species of crab {Medwus haswelli), although 

 coming from another and distant locality, were found to 

 bear similar or identical hydroids. 



Prof. Alcock has described the curious association 

 between a Hydroid {Stylactis minoi) and a small rock perch 

 {Minous inermis) ; but even more remarkable is Prof. 

 WiUey's case of barnacles growing on a sea-snake. His 

 figure, almost mediaeval at first glance, shows a bunch of two 

 kinds of barnacles {Lepas anserifera and Conchoderma 

 hunteri) attached to the end of an Indian Ocean sea-snake 

 (Hydrus platurus). The barnacles are not in any way para- 

 sitic, they are simply epizoic ; the free-swimming young 

 forms happened to fix themselves to the snake instead of 

 to a drifting spar. But it is interesting to notice that their 

 occurrence on snakes has been repeatedly recorded. To 

 the snake, one would think, they must prove themselves 

 a troublesome incubus, seriously impeding its movements. 



Some of the epizoic associations certainly become dan- 

 gerous to the bearer. Prof. Charles Chilton describes such 

 a case in the crab Paramithrax hngipes, which seems to be 

 almost invariably accompanied by specimens of the acorn- 

 shell Balanus decorus, growing on its carapace and some- 

 times becoming so large and numerous that they exceed in 

 size the body of the crab itself. The association was prob- 

 ably quite unimportant in its initial stages, but gradually, 

 as the cirripedes grew, they must have become inimical 



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