| OCR Text |
Show 504 PROF. J. E. DUERDEN ON CRABS [Dec. 12, to stimuli take place in a Melia deprived of its actinians as when they are present, but the empty chelipeds make no attempt whatever to grasp the source of irritation as in ordinary crabs. The movable joint (dactylopoclite) remains partly open, to about the same degree as when holding an actinian, and its use as an organ for direct protection or attack seems to have altogether disappeared. The responses of the chelipeds are manifestly so many instinctive reflexes on the part of the crab, directed towards the region whence the irritation comes, and are carried out independently of the presence or absence of the actinians. When the latter are in position, the reactions may be assumed to be aggressive or protective in their nature; while when taking place in the absence of the polyps they are of no protective value, as the claws are useless for grasping or seizing. Compared with most crabs, it would seem that there is in Melia a marked increase in the power of directive response on the part of the chelipeds, accompanied by a loss of activity on the part of the movable joint which ordinarily serves for aggressive purposes; moreover, the chelipeds as a whole are greatly reduced in size. F e e d in g R e a ct io n s . Under ordinary circumstances, the Melias were often seen transferring towards their mouth the debris occurring on the bottom of vessels or other objects over which they passed. This they accomplished by means of the maxillipeds, with the assistance of the first, second, and even third pair of ambulatory limbs, the first ambulatory pair being the most active. The debris was seized by the maxillipeds, and the nutritive particles were ingested, and the non-nutritive rejected and wafted away posteriorly. The chelipeds, whether empty or holding actinians, took no part whatever in the feeding processes. Even fragments of meat given directly to the crab were treated in the same manner; their passage to the masticatory appendages was effected by the first ambulatory limbs, always without the assistance of the chelipeds. As the actinians were dragged about from place to place, debris readily adhered to them by means of the viscid slime with which they were covered, and very frequently the first ambulatory limbs were applied to the polypal walls, and the latter Avere thoroughly cleansed from any adhering particles. The operation was carried out much in the same way as that by which the crab cleansed its own limbs. During the process the actinians were brought close to the mouth, and the debris removed was easily transferred thereto by the appendages. Some of the particles were nutritive, and there is no question that the crab will frequently secure food material thus mechanically adhering to the walls of the polyp. No instance was observed where the crab applied its appendages to the general body-surface of the polyps except when foreign partioles were adherent. During the cleansing treatment the |
