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Show of commensalism of such a nature, to conceive that a similar instinct has appeared suddenly in the case of two distinct species of crab, and its possession proved favourable to the survival of the individuals. Such an explanation may suffice until it can be put to experimental test, or until extended observations on the activities and structure of animals render the theory of mutation as plausible in the animal kingdom as from De Vries's work it is among plants. S u m m a r y . 1. The commensalism between the crab Melia tessellata and actinian polyps is not restricted to a single species of actinians. Of two crabs captured, one carried a Bunodeopsis in each claw and the other a Sagartia. 2. As regards the same crab the two actinian species are interchangeable, and the crabs will dislodge a small polyp of one species to take up a larger polyp of another (intelligent selection). 3. Apparently the crab is not aware of the presence of an actinian until it comes into tactile connection with it. Dislodgment of a fixed actinian is brought about by the insertion of the first ambulatory limb between the polypal base and the substratum. 4. The crab travels with the actinians expanded and directed forwards, sometimes waving them from side to side. When irritated it responds by moving its chelipeds towards the source of irritation, thereby placing the actinians in what may be considered as the most favourable aggressive or defensive attitudes. The crab reacts in the same manner, whether carrying the actinians or deprived of them. 5. Food given the polyps is abstracted by the crab by means of its first pair of walking-limbs, the stimulus to activity being derived from the diffusion of the meat juices. 6. In correlation with the commensal habits the crabs have no direct use of the chelipeds as aggressive or defensive organs, or for grasping objects other than the actinians, and the functions of the first ambulatory appendages are partly modified. 7. The commensal actinians present no structural or physiological modification compared with closely allied free species elsewhere. 8. The ccenobiotic habit seems to be necessary for the existence of Melia, though not for that of the actinians. 9. A second species of crab, Polydectus cupilifera, also bears an actinian, Phellia, in its chelipeds; specimens of the actinian are also found adherent to stones and coral rock in the natural habitat of the crabs. Thus in all probability a similar commensal habit has been acquired independently by two wholly distinct forms of crabs. 10. The advantages of the commensalism to the crab are (1) that 510 ON CRABS BEARING ACTINIANS IN THEIR CLAWS. [Dec. 12, |
