212 THE LIFE OF CRUSTACEA 



of the Coral, and that the irritation of its presence 

 causes the branches to grow up and surround it, 

 coalescing with each other to form a kind of cage, 

 and ultimately leaving only one or two small open- 

 ings. Through these openings water can enter to 

 enable the Crab to breathe, and no doubt food 

 particles find their way in, but it is not possible for 

 the Crab to leave its prison. The production of 

 these abnormal growths of the Coral is closely 

 analogous to the formation of " galls " on plants as 

 a result of the irritation set up by the presence of 

 insect larvae or other parasites, and it is not inappro- 

 priate, therefore, to speak of them as " Coral galls." 



The Medusas, or Jellyfishes, like other Ccelentera, 

 are provided with poisonous stinging cells, which, in 

 the larger species of our own seas, are powerful 

 enough to cause discomfort to bathers who come in 

 contact with them. The protection thus afforded is 

 no doubt of advantage to the little globular Amphi- 

 pods of the genus Hyperia (Fig. 67), which are almost 

 always to be found sheltering under the bells of the 

 larger Medusae. In what way the Amphipods escape 

 injury from the stinging cells of their host is not 

 known. 



In all the cases mentioned, the advantages of the 

 partnership seem to be all on one side, but there 

 are numerous instances in which both partners seem 

 to reap some benefit. A species of Hermit Crab very 

 common in moderately deep water on many parts of 



