THE WEB OF LIFE 289 



of the cases are difficult. Thus Dr. W. T. Caiman 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 (Medceus 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. 

 Willey'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 Pammithmx longipes, 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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