The diapophyses of the sacral vertebra carry no ribs, the ilia being attached to them directly. They are either cylindrical as in the Ranidae and Cystignathidae, or they are more or less dilated as in all the other families, most strongly in the Pelobatidae and the Aglossa. In some members of the large sub-family of the Cystignathidae the otherwise cylindrical diapophyses are slightly dilated.
Fig. 4.–Dorsal view of the sacral or ninth vertebra (9), with the attachment of the ilium, of (1) Rana temporaria, (2) Bufo vulgaris, showing the whole coccyx and pelvis, (3) Pelobates fuscus, as examples of cylindrical and of dilated sacral diapophyses. (About nat. size.) a, Acetabulum; c, coccyx; i, ilium; z, anterior zygapophyses.
The sacrum is formed by the ninth vertebra, but there are a few interesting exceptions. Pelobates, Pipa, and Hymenochirus possess two sacral vertebrae; and, neglecting individual abnormalities, these three genera form the only exception amongst recent Amphibia. In the three genera the coccyx is fused with the second sacral vertebra, and such a fusion occurs elsewhere normally only in Bombinator with its single sacral vertebra. The morphologically oldest condition is normally represented by Pelobates, the sacral vertebrae being the tenth and ninth. One case has been recorded by Boulenger of Bombinator pachypus "with eleven segments," the last carrying the ilium. Individual lop-sided abnormalities have been described in Bombinator and Alytes, where the right ilium articulated with the tenth, the left ilium with the ninth vertebra. This shifting forwards of the ilium to the extent of one metamere has been continued further in Pipa, in which the sacrum is formed by the ninth and eighth vertebrae, their diapophyses fusing on either side into extra broad wing-like expansions. In old specimens of Palaeobatrachus fritschi the seventh vertebra is in a transitional condition, the ilium being carried by the ninth and eighth, and slightly also by the diapophyses of the seventh vertebra; and in P. diluvianus the diapophyses of all these vertebrae are united into one broad plate to which the ilia are attached. Lastly, in Hymenochirus the first sacral is the sixth vertebra, and this creature has thereby reduced the pre-sacral vertebrae to the smallest number known.
This shifting forwards of the iliac attachment implies the conversion of original trunk into sacral vertebrae, and the original sacral vertebra itself becomes ultimately added to the urostyle. The second sacral, the tenth of Pelobates, the ninth of Pipa, and the tenth on the right side of the abnormal Bombinator, are still in a transitional stage of conversion. In Discoglossidae the tenth is already a typical post-sacral vertebra, and is added to the coccyx, but it still retains distinct, though short, diapophyses. In the majority of the Anura the tenth vertebra has lost these processes, and its once separate nature is visible in young specimens only. In Bombinator even the eleventh vertebra is free during the larval stage. In fact the whole coccyx is the result of the fusion of about twelve or more vertebrae, which from behind forwards have lost their individuality. We conclude that originally, in the early Anura, there was no coccyx, and that the ilium was attached much farther back; and this condition, and the gradual shifting forwards, supply an intelligible cause of the formation of an os coccygeum. The fact that the sacral vertebrae of the Anura possess no traces of ribs as carriers of the ilia, is also very suggestive. The ilia have shifted into a region, the vertebrae of which had already lost their ribs. By reconstructing the vertebral column of the Anura, by dissolving the coccyx into about a dozen vertebrae, so that originally, say the twenty-first vertebra carried the ilia, we bridge over the enormous gap which exists between the Anura and Urodela. That whole portion of the axial continuation behind the coccyx, more or less coinciding with the position of the vent, is the transitional tail.
The disappearance of both notochord and spinal cord, and the conversion of the cartilaginous elements into a continuous rod in the case of the os coccygeum, find an analogy in the hinder portion of the tail of Dipnoi and Crossopterygii, and in the tail-end of most Urodela, portions which are not homologous with the os coccygeum. The term urostyle should be restricted to such and similar modifications of the tail-end, and this latter happens to be lost by the Anura during metamorphosis.
Strictly speaking, or rather in anatomical parlance, the Vertebrate tail begins with the first post-sacral vertebra. In the Anura that portion of the whole tail has retained most cartilage, and has become the coccygeum, which is required as a "backbone" for the often enormous belly. This requirement is an outcome of the great shortening of the trunk proper (if the trunk be defined as ending with the pelvic region), and this shortening of the trunk is again intimately connected with the jumping mechanism, enlargement of the hind-limbs, elongation of the ilia, and throwing the fulcral attachment forwards as much as possible. The pre-acetabular ilio-sacral connection is carried to the extreme in the Anura.
The shoulder-girdle and "sternum" are more complete than in the Urodela, there being also a pair of clavicles, fused with the precoracoidal bars. The whole apparatus presents two types. In the arciferous type the coracoids and precoracoids retain a great amount of cartilage in their distal portions, and these cartilages (the epicoracoids of some authors) overlap each other movably on one another, the right usually lying ventrally upon the left. The epicoracoidal cartilage of each side, by connecting the distal end of the coracoid with the precoracoid of the same side, forms an arc, hence "arciferous." In the firmisternal type the epicoracoidal cartilages are much reduced, and, instead of overlapping, meet in the middle line and often fuse with each other, forming thereby a firm median bar, which connects the ventral ends of the precoracoids with those of the coracoids. This type is morphologically the higher and more recent, and passes in the larval stage through the arciferous condition. It is restricted to the Ranidae, Engystomatinae, and Aglossa. Although these two types afford an excellent distinctive character for the main divisions of the Anura, they are to a certain extent connected by intermediate forms in such a way, that, for instance, in Bufo and among Cystignathidae in Ceratophrys, the two opposite epicoracoidal cartilages begin to unite at the anterior end.
In many Engystomatinae the precoracoids together with the clavicles are much reduced, sometimes to thin ligaments, being in this case mostly curved back and lying closely against the coracoids; or they may be lost completely. Very rarely the precoracoidal bars are actually much stronger than the coracoids, and the median symphysial bar of cartilage is lost; this is the case in Hemisus.
The scapula is always large and curved into transverse, dorsally broadening blades, the dorsal greater portion of which, the so-called supra-scapula, does not ossify but calcifies.