Page:EB1911 - Volume 23.djvu/175

Rh form a bundle opposed to the rest; the fibulare and tibiale are fused into one bone; the fused fifth and fourth distal tarsals form a very large half-globular piece for the three outer toes, whilst the second toe is carried by the third distal tarsal, besides which there are three more small cartilages, one of which may be the displaced second tarsal or the still independent central. The tarsus of Sphenodon is like that of typical lizards, but none of its distal tarsals are fused on to metatarsals. The Crocodilian foot marks an advance. The astragalus is large, articulating well with tibia and fibula, and against the fibulare, which forms a typical, heel-shaped calcaneum. The fifth and fourth distal tarsals carry the fourth toe and the hook-shaped fifth metatarsal to which the fifth toe is reduced. The third, second and first distal tarsalia scarcely contain osseous nodules; they form together a wedge-shaped cartilaginous pad between the astragalus and the first and second toes. This attachment of the distal tarsals to the metatarsals reminds us of the Lacertilian condition, the result in either case being a still more marked intertarsal joint in addition to the cruro-tarsal.

Most well-footed reptiles retain all the five toes; only the crocodiles and a few tortoises have lost all the phalanges of the fifth toe. The phalangeal numbers are in the Lacertilia 2, 3, 4, 5 and 3 in the fifth toe; in chameleons 2, 3, 4, 4, 3; in most tortoises 2, 3, 3, 3, 2; but in Homopus, Pyxis and Cinixys 2, 2, 2, 2, 0; in the crocodiles 2, 3, 4, 4, 0. The embryos of crocodiles are said to be hyperphalangeal; i.e. as many as 7 phalanges on the fourth; 5 or 6 on the fifth finger; 6 on the fourth toe, and there are traces of the fifth toe. In the adult the fourth toe remains without a claw. Burrowing and living in sand, or humus, is in many lizards correlated with reduction of the limbs and their girdles. The vestiges of the hind limbs come to lie as near 34.—Vestiges of pelvic limb—1, of Lialis bartonii; 2, of Anguis fragilis; 3, of Amphisbaena fuliginosa. f, femur; il, ilium; ip, iliopectineum; p, pubis; t, tibia. the vent as possible. The reduction occurs in various families, independently. In most cases the fore limbs disappear first, but in the Amphisbaenidae, cf. Chirotes, and in the Tejidae, the reverse takes place. Whilst degeneracy of the shoulder-girdle is delayed long after the loss of the anterior limbs, that of the pelvic arch precedes the loss of the hind limbs. Cope has drawn up a tabular statistic of 35.—1, Vestigial pelvis and limb of Glauconia macrolepis. 2, The same parts of Boa (after Fürbringer). f, femur; il, ilium; ip, bone called “iliopectineum” by Fürbringer; p, pubis; t, tibia. the loss of digits, limbs and their girdles on pp. 202-3 of his work, Crocodiles, Lizards and Snakes of North America (Washington, 1900). The peculiar hind limbs of the Dibamidae are described in the article.

The majority of snakes have lost all traces of the limbs and their girdles except the so-called Peropoda (See : Classification). The vestiges of a Boa and Glauconia are shown in fig. 35.

The skin of reptiles is characterized by the strong development of its horny stratum; on the outside of it exists a thin cuticular or epitrichial layer. An important feature in most lizards and in the snakes is the existence of a “subepirdemoidal” or transitional layer which is produced by the migration of ectodermal cells into the cutis. The immigration takes place during the embryonic development, observed first by Kerschner, who, however, misinterpreted the process. Pigment cells, black chromatophores also, make their first appearance in the epiderm and then migrate into the transitional stratum, as has been first

correctly stated by F. Maurer. The horny stratum is shed periodically, several times during the year, and as one entire piece in snakes and a few lizards, e.g. Anguidae; in most lizards, chameleons, geckos and in Sphenodon the thin, transparent colourless layer comes off in flakes. In crocodiles it is not shed except for the usual Wear and tear, nor in tortoises, although in some e.g. Chrysemys, a periodical peeling of the large shields has been observed.

In all reptiles the cutis is raised into papillae, or folds. When the papillae are small the skin appears granular; when they are large, flat, mostly imbricating, they form scales; when they are very broad-based and still larger, they are called scutes or shields. The overlying epidermal covering partakes of these elevations, often e.g. in many snakes, with a very fine system of ridges of its own. Such a scale, cutis and horny sheath, may form spikes, or crests. They all have only basal growth. Thus, for instance, a shield of a tortoise-shell is a much fattened scale, or cone, with the apex more or less in the centre, surrounded by marginal ridges which indicate the continuous additional growth at the base. The central “areola” represents in fact the size of the shield at the time of hatching.

Of very common occurrence is the development of bone in the cutaneous portion of the scales; such osteoderms occur in many lizards, very strongly developed in the scutes of the crocodiles, especially on the back; they also occur in the skin of tortoises especially on their legs and on the tail, and they probably constitute the peculiar shell of Sphargis, the leathery turtle (see ). Sphenodon and chameleons are devoid of such osteoderms, in geckos they are likewise absent, but calcifications occur in their tubercular skin. A similar process seems to have produced the egg-tooth of crocodiles and tortoises (see under Teeth below). Calcareous deposits, or at least deposits of guanine and more commonly of carbonate of lime, play a considerable rôle in the skin of lizards and snakes. These waste products of the metabolism are always deposited within cells, and a favourite place is the subepidermal layer. In combination with superimposed yellow or red pigment, and with the black chromatophores as a foil, partial or complete screen to the light, as the case may be, these mineral deposits are to a great extent answerable for the colours and their often marvellous changes in the skin (see ).

Peculiar pits in the scales of snakes and crocodiles are described under Sense-Organs below.

The skin of reptiles is very poor in glands, but the few which exist are well developed. Crocodiles possess a pair of glandular musk bags which open by rather large slits on the under jaw, against the inner side of the jaw. Another pair of musk glands are the anal glands. During great excitement all these glands can be everted by the crocodiles. Sphenodon and snakes have only the anal pair. Water tortoises have inguinal glands, which secrete a strongly scented fluid, opening near the posterior rim of the bridge. Trionyx has additional glands opening near the anterior part of the plastron. Peculiar glandular structures are the femoral pores of many lizards. They lie in a line from the inner side of the knee to the anterior margin of the anal region, to which they are restricted in the limbless Amphisbaenidae. Each pore leads into a subcutaneous pocket, sometimes with slightly acinous side chambers, the walls of which produce a smeary, yellowish matter consisting chiefly of the débris of disintegrated cells which dries or hardens on the surface in the shape of a little projecting rod. They occur in both sexes, but are most active in males during the pairing season. Their use is unknown. It would be far-fetched to liken them to forerunners of the sebaceous portions of milk glands, although not so imaginary as to see in them and in the sensory pits of snake scales the forerunners of the mammalian hairs!

Claws, scarcely indicated in Batrachia, are fully developed in all limbed reptiles. The base is sunk into the skin like our own finger nails; the dorsal and ventral halves are differentiated into a harder, more curved dorsal sheath-like portion, and into the beginning of a sole, especially in crocodiles and in blunt-toed tortoises. The first claw to be reduced is that of