But this is not the only difference. The prawn’s gills are not brush-like, but are foliaceous. They are not trichobranchiæ, but phyllobranchiæ; that is to say, the central stem of the branchia, instead of being beset with numerous series of slender filaments, bears only two rows of broad flat lamellæ (fig. [68], C, C′, l), which are attached to opposite sides of the stem (C′, s), and gradually diminish in size from the region of the stem by which it is fixed, upwards and downwards. These lamellæ are superimposed closely upon one another, like the leaves of a book; and the blood traversing the numerous passages by which their substance is excavated, comes into close relation with the currents of aerated water, which are driven between the branchial leaflets by a respiratory mechanism of the same nature as that of the crayfish.
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.
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.
None of these statements apply to the crabs. In these {273} animals the abdomen is short, flattened, and apt to escape immediate notice, as it is habitually kept closely applied against the under surface of the cephalothorax. It is {274} not used as a swimming organ; and the sixth somite possesses no appendages whatever. The breadth of the cephalothorax is often greater than its length, and there is no prominent rostrum. In its place there is a truncated process (fig. [72], B, r), which sends down a vertical partition, and divides from one another two cavities, in which the swollen basal joints of the small antennules (2) are lodged. The outer boundary of each of these cavities is formed by the basal part of the antenna (3), which is firmly fixed to the edge of the carapace. There is no exopoditic scale; and the free part of the antenna (3′) is very small. The convex corneal surface of the eye appears outside the base of the antenna, lodged in a sort of orbit (or), the inner margin of which is formed by the base of the antenna, while the upper and outer boundaries are constituted by the carapace. Thus, while in all the preceding forms, the eye is situated nearest the middle line, and is most forward, while the antennule lies outside and behind it, and the antenna comes next; in the crab, the antennule occupies the innermost place, the antenna comes next, and the eye appears to be external to and behind the other two. But there is no real change in the attachments of the eye-stalks. For if the antennule and the basal joint of the antenna are removed, it will be seen that the base of the eye-stalk is attached, as in the crayfish, close to the middle line, on the inner side, and in front of the antennule. But it is very long and extends outwards, behind the antennule and the antenna; {275} its corneal surface alone being visible, as it projects into the orbit.