360 



COMPARATIVE ANATOMY 



Perennibranchiata, the most anterior (hyobranchial) cleft becomes 

 closed in Siren ; in Necturus, Proteus, and Typhlomolge, the most 

 posterior also disappears, while in the Derotremata (e.g. Am- 

 phiuma) only one remains, viz. that between the third and fourth 

 arches, and this not in all cases. In this respect, therefore, the 

 Myctodera are the most primitive, and there is every reason for 

 assuming that the Perennibranchiata, though retaining certain 

 larval characters, have been derived from caducibranchiate forms. 



In Anuran larvae, the three pairs of external gills are less 

 complicated than those of Urodeles (cf. Fig. 266), and are soon 

 replaced by internal gill-tufts situated in the three branchial 

 clefts. 1 By the growth of opercular folds, which contain no 

 skeletal parts, the external respiratory aperture of either side 

 becomes gradually reduced in size, and the two branchial chambers 



A 3 



D E 



FIG. 266. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE AMPHIBIAN 

 GILL. (Mainly after P. Clemens.) 



A, rod-like, unbranched, primitive form, retained in the adult of certain Anura 

 (e.g. Xenopus). B, form in which there is only a single series of branches 

 (Anura). C, feather-form, with two opposite series of branches (Derotremata : 

 this form is also primarily met with in the Gymnophiona and in embryos of 

 Myctodera). D, wedge-shaped, unbranched axis, on the lower border of 

 which the gill-filaments arise in rows (embryos of most Urodeles). E, leaf- 

 like, unbranched axis, on which the filaments increase in number, and are 

 arranged in four rows on the surface as well as on the edges (Axolotl-stage 

 of Amblystoma, Necturus). F, branched axis (Proteus, Siren). 



usually open eventually by a single aperture, which is situated 

 either in the median ventral line (Bufo, Bombinator) or laterally 

 (Rana). The larvae of the Gymnophiona also possess external gills, 

 which in Epicrium glutinosum, for example, are feather-like. 2 



1 The so-called internal gills of Anura are said to correspond to a series of 

 outgrowths from the bases of the external gills, and thus not to be comparable 

 with the endodermal gills of Fishes. 



2 The external gills of Amphibia present a great variety of form, often 

 resulting from adaptation. Thus, in the intra uterine larvae of the viviparous 

 Salamandra atra, they reach a length of 5-6 centimetres ; in Cecilia covn- 

 pressicauda they consist of two large, flattened, vascular folds, which apparently 

 cover the body of the larva like a mantle ; and in Notodelphys (Nototrema) 

 amongst Anuran s, in which the larvae undergo development in the pouch on 

 the back of the mother, they are bell-shaped and stalked. In certain other 

 Batrachians in which there is no free larval stage, it appears that respiration 

 may take place before hatching by means of the broad and vascular tail (Hylodes 



