1905.] BEARI^^G ACTINIANS IN THEIR CLAWS. 507 



capitulum) alone being naked. In aquaria they are very inactive 

 and do not readily expand. 



Polydectus was under observation for only two or three days, 

 and during most of that time it remained quiescent, hidden under 

 fragments of coral. It allowed itself to be pushed over the floor 

 of the vessel, making only a feeble attempt to escape, and showed 

 little or no activity with its chelipeds. If irritated, the chelae 

 were not directed against the source of the stimulus as in the case 

 of Jlelia. When the actinians were gently removed from the 

 claws and after a time again presented, the crab made no imme- 

 diate attempt to seize them. On the whole Polydectus proved 

 itself to be a most unsuitable crab for experimental studies. 



Interdependence of Crab and Actinian, 



Enquiry may now be made as to how far the crab and its 

 actinians are interdependent. Can the crabs maintain their 

 existence deprived of the actinians, and can the latter exist 

 separated from their captors ? Although a careful search was 

 made during three months' collecting, no free independent 

 examples of either Sagartia or jBitnodeopsis, the actinians com- 

 miensal with Melia, were met with, and neither Mobius nor 

 Borradaile speaks of finding such. There seems no reason, 

 however, why the actinians should not be able .to live separated 

 from the crustaceans. Compared with closely allied species 

 elsewhere, they present no modifications whatevei' which indicate 

 a correlation with the commensal habit. So far as the actinians 

 are concerned, their presence in the claws of the ciab seems of the 

 most incidental character, and it can scarcely be doubted that 

 ordinarily they are fixed isolated species, and may yet be found 

 as such either in the Hawaiian Islands or elsewhere. As regards 

 Polydectus and its associate Phellia, the latter certainly exists 

 independently of any commensalism, for all round the Hawaiian 

 Islands specimens of the sea-anemone are very numerous, attached 

 to the under surface of stones and coral blocks. These places 

 also constitute the habitat of the crab. In the case of the 

 actinians Sagartia and Adamsia, commensal with hermit crabs, 

 Faurot has shown experimentally that the polyps do not live 

 long when separated from theii' host ; but the relationship on 

 the part of the actinian is here much closer than in the polyps 

 simply held by Melia and Polydectus. In Sargartia palliata, at 

 any rate, the commensalism is correlated with a permanent 

 modification of form. 



The genus Btmodeopsis occurs also in the West Indian and 

 Mediterranean seas, where it lives in shallow water loosely 

 adherent to the leaves of the marine phanei-ogams Thalassia and 

 Ruppia. In these regions, however, it is never found associated 

 with crabs ; indeed, the genus 2Ielia is absent from the Atlantic. 

 A careful comparison of the external characters and internal 

 anatomy of the Hawaiian and West Indian species of Buna- 



