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Show 508 PROF. J. E. DUERDEN ON CRABS [Dec. 12, deopsis shows no important differences, and they may have to be regarded as one and the same species. Hence the commensal habit may be regarded as not essential to the life of Bunodeopsis, and the same can also be said of the Sagartia and Phellia. Of all actinians, members of the genus Bunodeopsis would appear to be the best adapted for the role of commensalism. They are active polyps with long tentacles which are usually expanded to their full extent, and in the absence of a sphincter muscle the column is incapable of overfolding the tentacles. The tentacles are provided with nematocysts of several sizes, and very large stinging-cells occur in the spheroidal outgrowths on the •column; and, lastly, the polyps are easily detached from any substratum to which they may be adherent. Were the commensalism of Melia restricted to Bunodeopsis, its suitability for such a relationship is so pronounced as to suggest more than a haphazard selection on the part of the crab. But when we consider that the same crab will also take up a Sagartia, it must be admitted that the wisdom of its selection is not so manifest ; for this form retracts readily on slight irritation, does not re-expand so freely as Bunodeopsis, and is usually very firmly attached to its substratum. Phellia, so far as observations upon its activities in aquaria go, seems even less desirable than Sagartia for the ccenobiotic habit. As regards the dependence of the crabs upon the actinians, the case seems much clearer. Owing to the absence of the usual junctions of the claws, the commensal habit would appear to be necessary to the existence of Melia. The claws, when deprived of the anemones, showed no power to grasp or seize other objects, not even when food was presented to them. The maxillipeds and ambulatory limbs transferred to the mouth any nutritive objects offered, but from their non-chelate character these appendages can be of little use in seizing or holding prey or warding off enemies. There is no question that the procuring of food by the crab itself would be very precarious were it not for the assistance of the actinians. Moreover, wherever met with, throughout its wide distribution, the crab is found to bear actinians. According to the observations of Mobius, all the specimens of Melia collected by him possessed polyps, though in Borradaile's experience they were sometimes absent from both claws, or from only one. It is to be expected that the crabs will occasionally lose their polyps, especially during ecdysis, and conceivably they may wander about for a time without meeting with others. If we attempt to estimate the advantages of the commensalism to the two organisms concerned, it must be admitted that the result seems to be entirely one-sided, and in favour of the crustacean. The experiments show that the reflexes of the crab are of such a nature as to result in the removal of any food or prey which the actinian may secure. The tentacles of the polyp move about freely and seize and retain organisms coming within their |
