324 ICHTHYOPSIDA. 



II. Amphibia (Frogs, Toads, and Newts). 



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

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

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

 noxious insects, not to mention slugs, snails, and other vermin. 



Amphibia resemble fishes in that they have gills always durmg 

 some part of their life; but, as a rule, these gills 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. 168) is not a true fin, for there are no fin-rays 

 such as we find in fishes. The limbs, in fact, approximate in 

 structure to those of the higher vertebrates, whilst the skull m\ 

 always articulates by two occipital condyles with the spinal 

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

 consists of three chambers — namel}^, two auricles and one 

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

 type, one wall 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 very marked, at others obscure, but always present, 1?!| 

 The larvae 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 jj I ; 

 pisciforme) of the Mexican 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 Amijhibia are the Urodda and 



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

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



