The Ypsiloid Apparatus of Urodeles

The ypsiloid apparatus is, with the exception of Cryptobranchus, confined to the suborder Salamandrida. It has arisen in response to the physical need of controlling the direction of the body in water through the adjustment of the relative buoyancy of the anterior and posterior ends. Its function is therefore closely correlated with the hydrostatic function of the lungs. 2. In origin the ypsiloid cartilage is independent of the pelvic girdle. Its stem arises as a chondrification of the linea alba of the somite immediately anterior to the pelvic girdle. The arms are more complex in origin since the process of chondrification involves not only the myocomma anterior to the above named somite but also the outer edge of the aponeurosis of the transversalis muscle. The Cartilago ypsiloides is therefore not homologous either with the Processus epipubicus of the lower Urodeles or with the marsupial bones of certain mammals. 3. In the Plethodontidœ and Desmognathidœ, in which the lungs have wholly degenerated, a correspondingly complete degeneration of the ypsiloid cartilage has occurred, although vestiges of the ypsiloid musculature remain to indicate the former possession of the apparatus. 4. This interpretation of the function of the ypsiloid apparatus throws some light upon the systematic position of certain of the lower Urodeles. The more obvious conclusions are: a. That forms with lungs but without vestiges of an ypsiloid apparatus, and with no evidence of degeneration in the pelvic region (e. g., Necturus) are neither degenerate forms, nor permanent larvæ of any of the Salamandrida. b. That the absence of the ypsiloid cartilage considered in connection with the absence of lungs in the case of Typhlomolge is in full accord with the conclusion [Emerson, 1905] that Typhlomolge is the permanent larva of some lungless salamander. c. That the presence of a functional ypsiboid apparatus in Cryptobranchus indicates that Cryptobranchus lies near the line of descent of the Salamandrida.