Although DeBeer (1937) shows in his figure of Hypogeophis (one of the Apoda) an operculum, this is apparently a mistake. The stapes has a large footplate, and its stylus articulates with the quadrate, but no true operculum or opercularis has been described in the Apoda. The facial nerve passes above the stapes. It does not seem necessary to regard the conditions in this order as related directly to those of either salamanders or frogs, but a reduction of the stapes comparable to that in salamanders has occurred.
The presence in both frogs and terrestrial salamanders of a special mechanism involving the opercularis muscle and an operculum cut out in identical fashion from the wall of the otic capsule behind the stapes seems to require some other explanation than that of a chance convergence or parallelism. Although the stapes and otic region are readily visible in a number of labyrinthodonts and lepospondyls, no indication of an operculum seems to be reported among them. But in the Triassic Protobatrachus (Fig. 1), which is unmistakably a frog in its skull, pelvis and some other features, Piveteau (1937) has shown, immediately behind the foot of the stapes, a small bony tubercle, which he and Watson (1940) designated opisthotic. Very clearly it served for insertion of a muscle, and it is equally clear that the bone is a reduced opisthotic, carrying the paroccipital process already mentioned as characteristic of it in some temnospondyls. Since the remainder of the posterior wall of the otic capsule consists of cartilage, meeting the exoccipital, it may be that the opisthotic becomes the operculum in frogs. Protobatrachus was too far specialized in the Anuran direction, although it still had a tail, and the forelegs and hind legs were nearly the same size, to be considered a possible ancestor of the Urodeles. But at one stage in the general reduction of the skull in the ancestry of both groups, a condition similar to that in Protobatrachus may have characterized the otic region, long before the Triassic.
In the argument thus far we have considered terrestrial, adult amphibians, since it is only in these that either the normal middle ear and tympanum, or the opercular apparatus, is present. But among the urodeles several neotenic types occur (this term applies also to the perennibranchs). For most of these there is nothing about the otic region that would be inconsistent with derivation, by neoteny, from known families in which adults are terrestrial; for example, Cryptobranchus could have had a Hynobiid-like ancestor. But this, as mentioned above, does not hold for the Proteidae, which possess an opisthotic of relatively large size, distinctly separate from the exoccipital and prootic. Either this bone is a neomorph, which seems improbable, or there has not been in the ancestry of this particular family an episode of reduction comparable to that seen in the terrestrial families, where there is an operculum instead of a normal opisthotic. Therefore the Proteidae probably are not derived from the general stem of other salamanders, but diverged sufficiently long ago that the bones of the otic region were reduced on a different pattern. They need not be removed from the order, but, in this respect, recognized as more primitive than any other existing Urodela or Anura. A recent paper by Hecht (1957) discusses many features of Necturus and Proteus, and shows that they are remote from each other; his evidence does not seem to prove, however, that they were of independent origin or that they need be placed in separate families.
VERTEBRAE AND RIBS
Development of the vertebrae and ribs of Recent Amphibia has been studied by Gamble (1922), Naef (1929), Mookerjee (1930 a, b), Gray (1930) and Emelianov (1936), among others. MacBride (1932) and Remane (1938) provide good summaries. In this section reference will be made to the embryonic vertebral cartilages by the names used for them in these studies, although the concept of "arcualia" is currently considered of little value in comparative anatomy.
Fig. 5. Development of Anuran vertebrae. Upper left, late tadpole of Xenopus laevis; lower left, same just after metamorphosis; upper right, diagram of general components of primitive Anuran vertebra. (After MacBride, 1932, Figs. 35, 38, 47D, respectively.) Lower right, section through anterior portion of urostyle, immediately posterior to sacral vertebra, in transforming Ascaphus truei (original, from specimen collected on Olympic Peninsula, Washington). All × 20 approx. For explanation of abbreviations see Fig. 3.
The centrum in Anura (Fig. 5) is formed in the perichordal sheath (Rana, Bufo) or only in the dorsal portion thereof (Bombinator, Xenopus). The neural arch develops from the basidorsal cartilages that rest upon, and at first are entirely distinct from, the perichordal sheath. Ribs, present as separate cartilages associated with the 2nd, 3rd and 4th vertebrae in the larvae of Xenopus and Bombinator, fuse with lateral processes (diapophyses) of the neural arches at metamorphosis, but in Leiopelma and Ascaphus the ribs remain freely articulated in the adult. Basiventral arcualia have been supposed to be represented by the hypochord, a median rod of cartilage beneath the shrinking notochord in the postsacral region, which at metamorphosis ossifies to produce the bulk of the urostyle. Fig. 5, lower right, a transverse section taken immediately posterior to the sacral ribs in a transforming specimen of Ascaphus, shows that the "hypochord" is a mass of cartilage formed in the perichordal sheath itself, and very obviously is derived from the ventral part of postsacral perichordal centra; there are, then, no basiventral arcualia, and the discrete hypochord shown in MacBride's diagram (Fig. 5, upper right) of a frog vertebra does not actually occur below the centrum, but only below the notochord in the postsacral region.
