URODELA. 189 



(Derotrema). Many, however, even completely lose the latter, and 

 show themselves by their whole organisation to be the highest mem- 

 bers of the order (Salamandrina). In the two first cases the vertebrae 

 are biconcave, like those of the Fishes, and enclose well-preserved 

 remains of the notochord. The fully-developed Salamandrina, on 

 the contrary, have vertebrae with an articular head in front and a 

 concavity behind (i.e., are opisthocoalous). 



The eyes, which are small and sometimes rudimentary, are placed 

 beneath the transparent skin, and except in the Salamandrina are 

 without distinct lids. In all cases the auditory organ is without a 

 tympanic membrane and tympanic cavity. The nasal apertures are 

 placed at the end of the projecting snout, and lead into slightly 

 developed nasal cavities, which communicate with the buccal cavity 

 by openings placed far forward in the roof of the mouth immediately 

 behind the maxillae. The buccal cavity is armed with small sharp 

 hooked teeth, which on the lower jaw are arranged in single rows, 

 but on the upper jaw and often on the palatine bone are in double 

 rows. Almost the whole lower surface of the tongue is attached to 

 the floor of the buccal cavity. 



The life history of the Axolotl, which was taken by Baird, Cuvier, 

 and others for the larva of a Salainandrine, is very remarkable. 

 According to the observations which were made by Dumeril in the 

 Jardin des Plantes at Paris, the young reared from the eggs of the 

 Axolotl under suitable conditions lose the gill tufts and develop 

 into a form which agrees with the Salamandrine genus Amblystoma, 

 while the specimens which were originally introduced from Mexico 

 preserve the Perennibranchiate form in the sexually adult condition. 

 Species of Triton also have occasionally been found with perfectly 

 developed gill tufts in the sexually adult state. 



Sub-order 1. Ichthyoidea.* 



Urodela with three pairs of external gills or without them, but with 

 persisting branchial aperture ; with fish-like biconcave vertebrae and 

 well-preserved notochord. 



The Ichthyoidea represent the lowest grade among the Urodela 

 with regard to their respiration, the structure of their skeleton, and 

 their whole organisation ; and to a certain exl/ent represent persistent 

 developmental stages of the Salamandrina. The eyes are small, and 



* Configliachi and Eusconi, " Del Proteo anguino di Laurenti," Paris, 

 1819. 

 Hyrtl, " Cryptobranchus japonicus," "Wien, 1868. 



