Fig. 5.–Ventral views of the shoulder-girdles of various Anura. (Slightly enlarged.) 1, Bombinator igneus, and 2, Bufo vulgaris, as examples of the arciferous type; 3, adult, 4, metamorphosing Rana temporaria showing change from the arciferous into the firmisternal type; 5, Hemisus guttatum; 6, Breviceps gibbosus; 7, Cacopus systoma. (5, 6, 7, after Boulenger.) Cartilaginous parts are dotted; ossified parts are left white. Cl, Clavicle; Co, coracoid; E, epicoracoidal cartilage; H, humerus; M, metasternum; O, omosternum; P, precoracoid; Sc, scapula; S.S, supra-scapula.
It is very doubtful if the Anura possess a true sternum, if by sternum we understand a medio-ventral apparatus which owes its origin to the ventral portions of ribs. The so-called sternal apparatus of the Anura consists of two pieces. One, anterior, variously named episternum, presternum, or omosternum, rests upon the united precoracoids and extends headwards, being either styliform or broadened out. Sometimes it is partly ossified, with a distinct suture at its base; this is the case especially in the Firmisternia; in many Arcifera the omosternum remains cartilaginous and is continuous, without a sutural break, with the cartilage of the precoracoids, indicating thereby its genetic relation to the shoulder-girdle. Hence omosternum is the preferable name. It is frequently much reduced, even absent, for instance in most Bufonidae and in the Engystomatinae. The posterior so-called sternal part may be termed metasternum. It forms the posterior counterpart of the omosternum. It is attached behind to the epicoracoidal cartilages, or fusing with them forms their posterior continuation. It appears mostly in the shape of a style, which is frequently ossified, and broadens out behind into a cartilaginous, partly calcified blade. In the Discoglossidae only it diverges backwards into two horns, assuming a striking resemblance to the typical xiphisternum of the Amniota. In young Anura the metasternal cartilage is intimately connected with the pericardium, an indication of its being derived not from ribs but from the shoulder-girdle.
The glenoid cavity is always formed by the coracoids and by the scapula, but the precoracoid often takes part in its formation, for instance in Bufonidae, Hylidae, and Discoglossidae.
In the fore-limb the humerus has a crest, stronger in the males than in the females; it assumes extraordinary strength in some Cystignathidae, notably in the male Leptodactylus. Radius and ulna are fused into one bone. The carpalia are originally nine in number: radiale, ulnare, two centralia, and five carpalia distalia, the fifth of which is reduced to a tiny nodule or to a ligamentous vestige. The primitive condition still prevails in the Discoglossidae. In most of the other Anura the fourth and third distal carpalia, in any case very small, fuse with the enlarged ulnar centrale; the radial centrale comes, in the Bufonidae and Pelobatidae, into contact with the radius, so that the forearm articulates with three elements as in the Urodela, but with this difference, that the intermedium of the Urodela has been lost by the Anura. There are five metacarpalia and five fingers, but the elements of the first or thumb are nearly vestigial, so that the pollux is reduced to one or two nodules, scarcely visible externally. The normal number of the phalanges of the second to fifth fingers is 2, 2, 3, 3. The distal phalanges are generally straight, either pointed or expanded or with Y or T-shaped ends; but in the Hylidae, in Hylambates amongst the Ranidae, and in Ceratohyla, one of the Hemiphractinae, the terminal phalanges are produced into curved claws which support the adhesive finger-discs. There are, however, many genera of different families, which possess finger-discs and have no claw-shaped phalanges. The Hylidae, and many of the climbing members of the Ranidae with adhesive discs, possess an extra skeletal piece intercalated between the last and last but one phalanges of the fingers and toes. This piece, a mere interarticular cartilage in Hyla, is in the following Raninae developed into an additional phalanx, so that their numbers are 3, 3, 4, 4 in the hand and 3, 3, 4, 5, 4 in the foot: Cassina, Hylambates, Rappia, Megalixalus, Rhacophorus, Chiromantis, Ixalus, and Nyctixalus. All the other Ranidae are without this additional phalanx, irrespective of the presence or absence or size of digital expansions.[[12]]
The pelvic girdle looks like a pair of tongs (see Fig. 4, p. [22]). The ilium is enormously elongated and is movably attached to the sacral diapophyses. This connection is always pre-acetabular in position. The ilium and ischium co-ossify completely, and make up nearly the whole of the pelvis; the pubis is very small, and remains cartilaginous unless it calcifies. It rarely possesses a centre of ossification, for instance in Pelobates, where the osseous nodule is excluded from the acetabulum, recalling certain Labyrinthodonta, whose ossa pubis likewise do not reach that cavity. The latter is open or perforated in young Anura and remains so in the Discoglossidae, but in the others it becomes closed up as in the Urodela. The ventral halves of the pelvis, besides forming a symphysis, closely approach each other, just leaving room for the passage of the rectum and the urino-genital ducts.
The hind-limbs are in all cases longer than the fore-limbs. The femur is slender, the tibia and fibula are fused into one bone. The tarsus is much modified by the great elongation of the two proximal tarsalia (there being no intermedium) into an astragalus and a calcaneum, both of which fuse together distally and proximally, or completely as in Pelodytes; in the latter case the limb assumes a unique appearance, since it consists of three successive and apparently single bars of nearly equal length. The other tarsal elements, especially the more lateral ones, are practically reduced to pads. The Anura have thereby acquired two well-marked joints, one cruro-tarsal, the other tarso-metatarsal; this shows a high stage of specialisation in comparison with the Urodelous and Stegocephalous type of still undefined joints.
The Anura possesses five well-developed toes with normally 2, 2, 3, 4, and 3 phalanges, and the rudiments of a sixth digit, the so-called prehallux, which consists of from two to four pieces, including the one which represents its metatarsal. This prehallux, as a vestige of a once better developed digit, is exactly like the elements on the radial side of the wrist, which, we are certain, are the remnants of a once complete finger, namely the pollex. The only weighty difficulty against its interpretation as a prehallux lies in the fact that hitherto no six-toed Stegocephali have been found; but the fact that there are no Stegocephali known with more than four fingers could be used as an argument against there being a pollex-vestige in recent Anura with just as little reason.
The skull of the Anura differs from that of the other recent Amphibia in the following features:–
The orbital region of the primitive cranium remains cartilaginous, but further forward the cranial cavity is closed by the unpaired sphenethmoid, which forms a ring round the anterior portion of the brain-cavity, hence called "os en ceinture" by some anatomists. The frontals and parietals fuse into one pair of fronto-parietal bones, and these again can fuse together in the middle line; as in Aglossa and Pelobates. The palatal portion of the palato-quadrate cartilage is complete, reaching forwards to the sides of the ethmoid region. The curved arch, formed by this cartilage, is covered by the following bones: (1) the quadrato-jugal, reduced to a thin splint which connects the quadrate and squamosal with the posterior end of the maxilla; (2) the pterygoid, always strong, extending from the distal inner corner of the quadrate to the maxilla, sometimes also to the palatine, and with a broad, median process to the parasphenoid, this process covering ventrally most of the otic region; (3) the palatines, which vary considerably in shape and size; they are placed transversely and meet in the middle line; in Bombinator and Pelodytes they are absent.