Coming now to the true crabs--the _Brachyura_, or Short-tailed crustaceans, as sometimes distinguished from the _Anomura_--we find quite a variety of interesting creatures, many species of which are always within the reach of the collector at work between the tide-marks. In all these the abdomen is only slightly developed, and is never used in swimming, being permanently folded beneath the thorax. This portion of the body, however, is usually very distinctly segmented, and if it be lifted from its position it will be found that some of the segments bear appendages corresponding with the swimmerets of the lobster. It is also wider in the female than in the male, and crabs of the former sex may often be found during the summer with the abdomen more or less depressed, and the space beneath it quite filled with eggs. The upper surface of the carapace of crabs is often very distinctly grooved, and it is interesting to note that these features of the exo-skeleton are not merely of external significance, for they usually correspond in position with various internal structures, some of them denoting the areas of the insertions of important muscles, and others enclosing the regions of certain of the internal organs. It will be noticed, too, that the carapace, which in lobsters is often less than half the length of the body, covers the entire body of the crab, except, perhaps, a very small linear portion between the bases of the last pair of legs, where the first part of the segmented abdomen is visible from above. The true crabs of our seas may be divided into four groups, as follow: 1. _Oxystomata_, or Pointed-mouthed Crabs; 2. _Oxyrhyncha_, or Pointed-beaked Crabs; 3. _Catometopa_, with forehead turned downwards; and 4. _Cyclometopa_, or Round-headed Crabs; and we shall briefly observe some of the more conspicuous and interesting species in the order of the tribes as just given. The first division is not well represented in our seas, the principal species being the Nut Crabs and the Long-armed Crab, all of which may be distinguished by the peculiar arrangement of the foot-jaws, which, when closed, form a triangle with an acute angle turned towards the front. The Nut Crabs are mostly small; and, since they generally inhabit deep water, are not commonly seen on the shore; but perfect specimens may sometimes be found among the contents of fishes’ stomachs. They derive their name from the nature of the carapace, which is of a rounded form and very hard and strong. Pennant’s Long-armed Crab (_Corystes Cassivelaunus_) may commonly be seen entangled among fishermen’s nets, but is not often seen on the shore at low tide. Its carapace is very convex above, with three sharp spines on each side, and the grooves are so arranged as to suggest the appearance of a face. Our illustration represents the female, but the ‘arms’ of the male are very much longer than those of this sex.