CRUSTACEANS it quite clearly from the marble crab which has a smooth carapace with patterns worked in buffs and reddish browns, and also from P. depurator (Linn.), the cleanser swimming crab, which has the carapace pale reddish brown in colour and irregularly granular in texture. Of this cleanser or port-scavenger Leach says, ' It is well known to the fisher- men under the name of flying crab, and is supposed by them (though erroneously) to destroy oysters, by insinuating its flattened foot into the shells when the animal opens for food.' ' The terminal joint of the fifth foot is here, as in P. holsatus, broadly oval and smooth, not ridged as in P. puber. P. pusillus, the dwarf swimming crab, is much smaller than the other species of the genus here recorded. In the same section of Cyclometopa, or round-fronted crabs, is included another family, the Corystidae, represented in Great Britain by the singular masked crab, Corystes cassivelaunus (Pennant). The mark- ings on the carapace, especially if a little accentuated by pen or pencil, give the effect of human features, and to this Bell's English name for it is due, the same idea having been previously conveyed by the Latin name, personatus, which Herbst bestowed upon the female. The chelipeds of the male are so very much longer than those of the other sex, that the earlier students may be excused for having thought that they had a specific distinction to deal with. In the Corystidae as in the Cancridae the first antennae are longitudinally folded, but whereas in Cancer pagurus the carapace is much broader than long, and the second antennae are inconspicuous, here the carapace is much longer than broad and the second antennae are as long as or longer than the carapace. Moreover by a geniculation of the peduncles these external antennae have their flagella brought close together in the longitudinal axis of the animal as if they were the inner instead of the outer pair. The masked crab has the habit of burying itself perpendicularly tail foremost in the sand at the bottom of the sea during the day time. While in this position it naturally finds the ordinary mode of respiration in vogue with the Brachyura by no means convenient. For as a rule the current of water which bathes the branchiae or gills enters the branchial chambers below the branchi- ostegite or branchial-cover, and comes out in front by the apertures at the sides of the mouth. But Corystes in its burrow being beset by sand, except for the little tube which it forms with its hairy second antennae stretched upward in juxtaposition, can only enjoy the current by reversing it. Mr. Walter Garstang, who by help of an aquarium has carefully watched this creature's behaviour, thus summarizes the matter : ' The elongation of the antennae, and the arrangement of the hairs upon them, the double bend of their basal joints, the structure of the parts bounding the prostomial chamber, and the arrangement of hairs upon them, are characters which in conjunction with the reversal of the respiratory current, adapt the respiratory mechanism of the crab in a remarkably complete manner to the arenicolous mode of life. The antennal tube enables the crab to draw its supplies of water directly from ' Edinb. Encycl. vii. 390 (1813). I 241 31
