508 PROF. J. E. DUERDEN ON CRABS [DeC. 12, 



cleopsis shows no important difi'erences, and they may have to be 

 regarded as one and the same species. Hence the commen.sal 

 habit may be regarded as not essential to the life of Buno- 

 deopsis, 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 

 substi'atum to which they may be adherent. Were the com- 

 mensalism 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 ci-ab. But when we 

 consider that the same crab will also take up a iSagartia, 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 Bitnodeopsis, and is usually very firmly 

 attached to its substratum. Phellia, so far as observations 

 upon its activities in aquaria go, seems eA^en less desirable than 

 Sagartia for the coenobiotic 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 eveiTi when food was presented to them. The maxillipeds and 

 ambulatoiy limbs transferred to the mouth any nutritive objects 

 offered, but fi^om 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 otheis. 



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 crus- 

 tacean. 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 



