AMPHIBIA. 141 



those instances in which respiration is mainly carried on by their means. 

 The external gills of Elasmobranchii are probably, as stated in -a previous 

 chapter, examples of secondarily developed structures, which have been 

 produced by the same causes as the enlarged gills of Alytes, Notodelphis, 

 etc. 



Urodela. Up to the present time complete observations on 

 the development of the Urodela are confined to the Myctodera 1 . 



The early stages are in the main similar to those of the 

 Anura. The body of the embryo is, as pointed out by Scott 

 and Osborn, ventrally instead of dorsally flexed. The metamor- 

 phosis is much less complete than in the Anura. The larva of 

 Triton may be taken as typical. At hatching, it is provided 

 with a powerful swimming tail bearing a well-developed fin : 

 there are three pairs of gills placed on the three anterior of the 

 true branchial arches. 



Between the hyoid and first branchial arch, and between the 

 other branchial arches, slits are developed, there being four slits 

 in all. At the period just before hatching, only three of these 

 have made their appearance. The hyomandibular cleft is not 

 perforated. Stalked suckers, of the same nature as the suckers 

 of the Anura, are formed on the ventral surface behind the 

 mouth. A small opercular fold, developed from the lower part 

 of the hyoid arch, covers over the bases of the gills. The 

 suctorial mouth and the provisional horny beak of the Anura 

 have no counterpart in these larvae. The skin is ciliated, and the 

 cilia cause a rotation in the egg. Even before hatching, a small 

 rudiment of the anterior pair of limbs is formed, but the hind- 

 limbs are not developed till a later stage, and the limbs do not 

 attain to any size till the larva is well advanced. In the course 

 of the subsequent metamorphosis lungs become developed, and 

 a pulmonary respiration takes the place of the branchial one. 

 The branchial slits at the same time close and the branchiae 

 atrophy. 



The other types of Myctodera, so far investigated, agree fairly with the 

 Newt. 



The larva of Amblystoma punctatum (fig. 84) is provided with two very 



1 The recent observations on this subject are those of Scott and Osborn (No. 114) 

 on Triton, of Bambeke (No. 95) on various species of Triton and the Axolotl, and of 

 Clark (No. 98) on Amblystoma punctatum. 



