These last three, therefore, are _sense-appendages_. Some writers make this last pair special organs, not homologous with appendages. [Illustration: FIG. 29.--Appendages of a prawn (after Cuvier).] [Illustration: FIG. 30.--Appendages of Nebalia.] For the sake of greater distinctness, we give the whole series of these appendages in one of the higher forms, viz., the prawn (Palemon, Fig. 29, and in one of the lower forms, Nebalia, Fig. 30). [Illustration: FIG. 31.--Vibilia, an amphibod crustacean (after Milne Edwards).] That these are really homologous parts is further shown by the fact that in the case of other crustaceans, such as limulus, the same appendages, i. e., the appendages of the same body segments, which in the cases before mentioned are used as feet, become swimmers, while the appendages corresponding to jaw-feet become walkers; and even what corresponds to antennæ or sense-appendages, may, as in branchippus, become powerful claspers. Finally, in all the lowest crustaceans, the identity is evident, because all the segments and their appendages are much alike in form and function (Fig. 31). [Illustration: FIG. 32.--Lithobius forcipatus (after Carpenter).] We have taken examples from near the middle of the articulate scale, because, as already stated, both the essential identity and the adaptive modifications are easily traced. If we go downward in the scale, the structure becomes more and more generalized, and the rings and appendages become more and more alike (Fig. 31), until in the most generalized forms we have only a series of similar rings, with similar pairs of appendages, except some necessary modifications to form the head and tail. This is well shown in the centiped (Fig. 32), and still better in marine worms (Fig. 33). In some marine worms the slight modification to form the head takes place under our very eyes. These often multiply by dividing themselves into two. When they do so, they make a new head and new tail by slight modification of segments and appendages (Fig. 33). [Illustration: FIG. 33.--Syllis prolifera.] If, on the other hand, we go up the scale, we find adaptive modifications obscuring more and more the simple and obvious identity of parts, until finally the identity can not be recognized without extensive comparison in the taxonomic series and study of embryonic conditions. In crabs--which is a higher form than cray-fish--the tail or abdomen seems to be wanting, but is only very small and bent under the body and thus concealed. In all essential respects the structure is precisely like the cray-fish. In fact, in the embryo, we trace the one form into the other; for the crab is at first a long-tailed crustacean (Fig. 34). [Illustration: FIG. 34.--Development of Carcinus mœnas. A, zoæa stage; B, megalopa stage; C, final state (after Couch).]