Ixvi Introduction. 



the larvae having in some cases shed their external branchiae 

 previously to birth. The ova are small, the yolk undergoes nearly 

 complete segmentation. With a few exceptions^ the Amphibia are 

 oviparous. In every case^ except possibly that of tlie Caecil'ia 

 compressicauda, the embryos very shortly after hatching develope 

 branchiae, or, as in the case of Notodelphys, structures equivalent 

 to them. 



Existing Amphibia are divisible into three orders. In the most 

 highly organized of these, the swimming tail is discarded in the 

 course of metamorphosis as well as the gills, and they are thence 

 called Anura ; in the second order, thence called ' Urodela ' the tail 

 is retained, whilst the gills are, in the sub-orders, Salamandrina 

 and Derotremata, deciduous ; and in the Peremiibranchiata, retained 

 permanently. In these two orders limbs are developed, at least on 

 the pectoral arch ; but a third order, that of the GymnopTiiona, 

 represented by the single family, Caeciliae, is constituted by 

 Amphibia in which, though the gills are deciduous, no limbs 

 are developed, and the body remains serpentiform. 



In development, the body cavity is not formed apart from and 

 around the yolk sac, but the intestine is formed, as in Amphioxus 

 and the Cyclostomi, by a process of invagination, beginning from 

 without at a spot corresponding with the situation of the future 

 anus. The oral opening is not formed when the embryos are first 

 set free from the egg. The Anura and the Caducibranchiate 

 Urodela have two sets of gills, an external set of three pairs, which 

 is soon lost, and in the land Salamanders partly or wholly before 

 the end of intra-uterine life ; and an internal set. After the dis- 

 appearance of the external set an opercular fold, in which however 

 in no Amphibia are bones ever developed, forms over the internal 

 gills, and within the branchial cavity thus produced the anterior 

 extremities first bud forth. When in the course of metamor- 

 phosis the gills disappear, the continuity of the circulation is main- 

 tained, or, in other words, the primitive continuity of the proximal 

 or cardiac with the distal or dorsal elements of the aortic arches 

 is re-established, by the expansion in calibre of a branch of ana- 

 stomosis, which, whilst the branchiae were functionally active, 

 connected the efferent directly with the afferent branchial trunks, 

 but was itself at that time functionally insignificant. The oper- 

 cular structures close up the visceral fissures, except in the Dero- 



