Different as these phyllobranchiæ of the prawns are in appearance from the trichobranchiæ of the preceding _Crustacea_, they are easily reduced to the same type. For in the genus _Axius_, which is closely allied to the lobsters, each branchial stem bears a single series of filaments on its opposite sides; and if these biserial filaments are supposed to widen out into broad leaflets, the transition from {272} the trichobranchia to the phyllobranchia will be very easily effected. The shrimp (_Crangon_) also possesses phyllobranchiæ, and differs from the prawn chiefly in the character of its locomotive and prehensile thoracic limbs. * * * * * There are yet other very well-known marine animals, which, in common appreciation, are always associated with the lobsters and crayfishes, although the difference of general appearance is vastly greater than in any of the cases which have yet been considered. These are the Crabs. In all the forms we have hitherto been considering, the abdomen is as long as, or longer than, the cephalothorax, while its width is the same, or but little less. The sixth somite has very large appendages, which, together with the telson, make up a powerful tail-fin; and the large abdomen is thus fitted for playing an important part in locomotion. Again, the length of the cephalothorax is much greater than its width, and it is produced in front into a long rostrum. The bases of the antennæ are freely movable, and they are provided with a movable exopodite. Moreover, the eye-stalks are not inclosed in a cavity or orbit, and the eyes themselves appear above and in front of the antennules. The external maxillipedes are narrow, and their endopodites are more or less leg-like. [Illustration: FIG. 72. _Cancer pagerus_, male (1/3 nat. size). A, dorsal view, with the abdomen extended; B, front view of “face.” _as_, antennary sternum; _or_, orbit; _r_, rostrum; 1. eyestalk; 2. antennule; 3. base of antenna; 3′, free portion of antenna.]