leg. The endopodite consists of five primitive joints, which are well developed and form the ordinary claw; these five parts are the ischiopodite (is.), the mesopodite (me.), the carpopodite (ca.), the propodite (pr.), and the dactylopodite (da.). The exopodite is missing. No vestige or rudiment of it is to be found in any phase of the development of the crayfish. In the lobster, however, which is closely allied to the cray-fish, the exopodite is still to be found during the larval period. The third fundamental part of the primitive member persists also in prawns throughout the entire period of life, but the organ is very small. At the extremity of the first and second pairs of walking legs there is an apparatus consisting of a fixed part-an elongation of the protopodite-and of a moveable part-the dactylopodite. This furnishes the walking leg with a prehensile organ which is well developed in the first pair of walking legs, and which is enormously increased in the true claws. In this evolution degeneration is exhibited by the disappearance of a joint, for in these appendages the basipodite and the ischiopodite are immovably united. This morphological degeneration corresponds to a functional change in the appendage. So long as the claw was used for locomotion a joint at this point was indispensable for progression. It is this joint which, in six or eight-footed beasts allows of the horizontal motion of the member necessary for locomotion, which in six-footed beasts results from the general 20 en structure of the parts and their auricular combinations. On the claw becoming prehensile, the joint hinge consolidates, the lever thus becoming much stronger and permitting the claw to be used to greater advantage.1 It now remains to examine the appendages situated in front of the maxillipedes, i.e. ba SC the appendages of the head. Fo. 12. — Astacus fluviatilis. In the second maxilla (fig. 12) a special transformation may be observed. The coxopodite Second left maxilla (1,5/1): co, coxopodite, and ba, basipodite, forming pr, protopodite; en, endopodite; sc, scaphog nathide (Huxley). (co.), and the basipodite (ba.), are flat plates; the endopodite (en.), which is small and undivided, exhibits signs of degeneration in its size, and in the absence of all articulations. The exopodite, according to some authorities on the FIG. 13.-Astacus fluvia- subject, no longer exists, while illa (1,5/1): co, coxo- according to others it constitutes tilis. First left max podite, and ba, basi protopodite; en, endopodite (Huxley). podite, forming pr, with the epipodite (the analogous part to that which we regard as representing the gill in the maxillipede), a large peculiarly-shaped blade, the scaphognathide (sc.). In the first maxilla (fig. 13), a partial degeneration of the organ is very marked: the exopodite and 1 See J. Demoor, Recherches sur la Marche des Crustacés (Arch. de Zool. exp. et gen., 2o serie, t. iv., 1891). T. List, Bewegungsapparat des Arthropoden, 1. Theil, Astacus Fluviatilis. Morphol. Jahrbuch., xxii. Bd. 3. Heft, 1895. the epipodite are missing; the endopodite is reduced to a mere unjointed stem, and only the protopodite retains its two normal component parts. The mandibles (fig. 14), the appendages of the fourth segment, are modified entirely for mastication. They consist of a strong transverse piece (pr.) provided at the extremity with an inner surface (st.) for grinding and sawing, and of a three-jointed piece (en.) with bristles which point outwards. The first piece is the result of the modification of the two parts of the protopodite, which have united to form the organ of mastication; the second, which represents the endopodite, is the feeler, an organ of sensation. All the other parts of the appendage have disappeared. The antenna (fig. 15), which is a tectile apparatus, is formed of two parts representing the segments of the protopodite (pr.). The long-ringed process is the endopodite, while the lateral scale of the antenna represents a much reduced exopodite (ex.). The antennule (fig. 16) consists of a protopodite (pr.), furnished with an annulated endopodite (en.) and exopodite (ex.). The eye-stalk (fig. 17) consists of a two-jointed protopodite (pr.). This is all that remains, the endopodite and the exopodite being absent altogether. pr This examination of the appen- FIG. 17.—Astacus fluvia dages of the cray-fish clearly shows that all fresh adaptation in the tilis. Left eye-stalk (1,5/1): pr, protopodite (Huxley). appendage entails the modification of some parts and the degeneration of others. In each case evolution is accompanied by degeneration. § 3. Transformation of homologous organs in individuals of different species. The limitations of our present knowledge make it difficult to determine definitely the origin of limbs among vertebrates, but they are universally supposed to have developed from the lateral folds which still persist in Amphioxus, and which probably existed in the ancestors of vertebrates.1 1 A. Morphological proofs: (a) Lateral folds of amphioxus. (b) Identity of the skeletons of paired and unpaired fins. B. Embryological proofs: (a) Continuous lateral folds in the embryoes of fish. At first the only skeleton of these lateral rods consisted of parallel rods of a tough material. By the transformation of these rods, the skeleton of the limbs of vertebrates was ultimately formed. The skeleton of limbs then consisted originally of a certain number of parallel rods. One of these rods, lying in the long axis of the future limb, became longer than the others, while the neighbouring rods began to slant, so that those nearest to the elongated rod spread out like a fan, and gradually moved outwards along the principal rod. This phenomenon was repeated several times, so that eventually those rods nearest to the principal rod passed towards the free end of it, and as the others followed in the same direction, the fin finally acquired a feather-like structure. This transformation of the continuous folds into limbs a progressive transformation, since new and more perfectly adapted organs were formed-was accompanied by degeneration, for a considerable part of the folds disappeared. In the same way, although the transformation of the parallel rods into bipinnated fins constituted a development, C. Palæontological proofs: (a) The series of paired spines in Diplacanthus and in D. Philogenetic proof: The necessity of the bifurcation of the E. Physiological proof: The lateral folds are the undifferentiated condition of organs of equilibrium in modern fish. |