The hyoid apparatus is still very primitive in many, especially in larval, Urodela. Besides the hyoid there are as many as four pairs of branchial arches, which, however, decrease in size and completeness, so that the last two have lost their connection with the median copular piece, and become attached in various ways to the second branchial arch. This is the arrangement apparently in all larvae, but four pairs of branchials persist in the adult Siren, Amphiuma, and Cryptobranchus alleghaniensis. The whole branchial apparatus is reduced to three pairs of arches in Necturus and Proteus, to two in the adult Cryptobranchus japonicus and in the Salamandridae. Of considerable interest is the vestige of a fifth pair of arches in the larvae of Triton and Salamandra, in the shape of a pair of tiny cartilages, which lie in front and on each side of the opening of the trachea, and give rise to the formation of the laryngeal cartilages, better developed in the higher Vertebrata.
The following are noteworthy characters of the skull of Urodela. The articulation of the skull with the vertebral column is not always effected entirely by the two condyles of the lateral occipital bones, but the median basal cartilage often possesses a pair of facets for the odontoid-like process of the first vertebra; such additional facets are perhaps best developed in Cryptobranchus and in the Salamandrinae.
The middle portion of the primitive cranium, from the exit of the optic nerve to the ethmoid cartilage, is formed by a pair of separate bones, the orbito-sphenoids. The parietal and frontal bones remain separate. One or more periotic bones exist, besides the prootic, in the aquatic families.
A pair of prefrontal bones is present in most Salamandridae, e.g. Salamandra, Triton, Amblystoma, especially in the larva, and in Cryptobranchus; these bones are absent in Amphiuma, Necturus, Proteus, and Siren.
Fig. 3.–Skulls of various Urodela. 1, Salamandra maculosa, ventral view, and 2, dorsal view; 3, Axolotl stage of Amblystoma; 4, adult stage of Amblystoma; 5, Salamandrina perspicillata (after Wiedersheim); 6, Salamandra maculosa, dorsal view of the lower jaw. A, Articulare; C1, C2, outer and inner occipital condyles; Ch, choana or posterior nasal opening; d, dentary; E, ethmoid; F, frontal; LO, lateral occipital; M, maxillary; N, nasal; No, nostril; OS, orbito-sphenoid; P, parietal; Pf, prefrontal; Pl, palatine; Pm, premaxillary; Po, prootic; PS, parasphenoid; Pt, pterygoid; Q, quadrate; S, angulo-splenial; Sq, squamosal; St, stapes; Vo, vomer; II, VII, X, exits of the optic, facial, and glosso-vagus nerves.
The lacrymalia are still separate in some Amblystomatinae, e.g. Ranidens and Hynobius. A pair of nasalia are generally present, but are absent in Necturus, Proteus, and Siren. The parasphenoid is furnished with teeth in the Plethodontinae and Desmognathinae.
Separate palatine bones exist in Necturus and Proteus, and in the larva of Amblystoma, but in the adult form they fuse with the vomers, producing the vomero-palatines characteristic of the majority of Urodela.
The pterygoid bones are most fully developed, so as to reach the vomero-palatines, in the Amblystomatinae, in Necturus, and in Proteus; they are reduced, so as to leave a gap, in Cryptobranchus, and still more in the Salamandrinae; they are absent in Amphiuma and in Siren.
The quadrates are directed forwards in Necturus, Proteus, and Siren, while in the other Urodela they extend transversely and almost horizontally. The hyomandibular remnant, the so-called operculum, is small, and forms a plate which fits into the fenestra ovalis, extending as a ligamentous process upon the quadrate.