354 ICHTHYOPSIDA. 



II. iVxpHiBLA {Frogs, ToaiU, and Xeais). 



Amphibia include the Frogs, Toads, Water - Newts, &c. 

 These must claim our attention for a while, as all three are of 

 much service to us as a means of keeping in check many noxious 

 insects, not to mention slugs, snails, and other so-called vermin. 



Amphibia resemble fishes in that they always have gills during 

 some part of their life ; but, as a rule, these gUls are not 

 persistent. On the other hand. Amphibia have generally lungs 

 in the adult form, and their limbs are never in the form of fins 

 as in the Pisces ; even the median fin-like structure seen in 

 Salamanders and Newts (fig. 201) is not a true fin, for there are 

 no fin-rays such as we find in fish. The limbs, in fact, approxi- 

 mate in structure to those of the higher vertebrates, whilst the 

 skull always articulates by two occipital condyles with the spinal 

 column. The heart also differs from the heart of fish, for it 

 consists of three chambers — namely, two auricles and one 

 ventricle.^ On dissecting a frog, which can be taken as the best 

 type, one will at once observe that the rectum, ureters, and the 

 ducts of the reproductive organs open into one common chamber, 

 the so-called cloaca. 



Lastly, the Amphibia develop by a metamorphosis, some- 

 times \erj marked, at others obscure, but always present. 

 The larvfe are provided with gills, which usually disappear, 

 giving place to lungs, in the adults. Some Amphibia do retain 

 the gills persistently, such as the remarkable Axolotl [Siredon 

 j)iscif(iniii') of the jMoxican lakes, which may attain the adult 

 stage whilst still retaining its gills. In captivity, as the author 

 has found in two cases, the Axolotl may lose its branchiae and 

 become terrestrial under forced circumstances ; whilst other ova 

 of a similar brood kept at the Brighton Aquarium remained with 

 persistent gills. 



The two chief groups of Amphibia are the Urodela and Amnira. 



1 The Dipnoid fish have a small second auricle. These so-called " Mud- 

 fish " of Africa and S. America are transitional between Fish and Amphibia. 



