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Show 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 cheke were not directed against the source of the stimulus as in the case of Melia. When the actinians were gently removed from the claws and after a time again presented, the crab made no immediate attempt to seize them. On the whole Polydectus proved itself to be a most unsuitable crab for experimental studies. I nterd e p en dence of C rab a n d A c t in ia n . 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 Bunodeopsis, the actinians commensal 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 whatever which indicate a correlation with the commensal habit. So far as the actinians are concerned, their presence in the claws of the crab 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 their 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 Bunodeopsis occurs also in the West Indian and Mediterranean seas, where it lives in shallow water loosely adherent to the leaves of the marine phanerogams Thalassia and Ruppia. In these regions, however, it is never found associated with crabs; indeed, the genus Melia is absent from the Atlantic. A careful comparison of the external characters and internal anatomy of the Hawaiian and West Indian species of Buno- 1905.] BEARING ACTINIANS IN THEIR CLAWS. 5 0 7 |
