226 VERTEBRATE ANIMALS. 



times retained throughout life. Most Amphibians, therefore, 

 are to a greater or less extent amphibious, that is to say, 

 more or less capable of living indifferently .either on land or 

 in the water. In the majority of cases, the gills arc external, 

 placed on the sides of the neck, and not contained in a special 

 cavity, thus differing altogether from the gills of fishes. In 

 the Frogs and Toads, and in some others, there are two sets 

 of gills, one external and the other internal, of which the for- 

 mer is soonest lost. The lungs of the Amphibians never attain 

 a very high state of development, and, in those forms in which 

 the gills are retained throughout life, the chief business of 

 respiration appears to be carried on by the gills. In accord- 

 ance with the changes in the respiratory process, correspond- 

 ing alterations take place in the blood-vessels. With the 

 development of the lungs, the vessels which carry blood to 

 them (the pulmonary arteries) increase in size, while the 

 branchial vessels which carry the blood to the gills undergo a 

 proportionate diminution. At first, the condition of the circu- 

 lation is very much the same as it is in fishes, but ultimately 

 it becomes nearly the same as in the true reptiles. 



The Amphibia are divided into three living and one ex- 

 tinct order, as follow : 



1. Ophiomorpha. 



2. Urodela. 



3. A.noura. 



4. Ldbyrintliodontia. 



ORDER I. OPHIOMORPHA (Gr. ophis, a serpent ; and morphe, 

 form). This order is an extremely small one, and, as its name 

 implies, it comprises certain snake-like Amphibians. The order 

 includes only the curious animals known as Ccecilice, which are 

 found in Java, Ceylon, South America, and Guinea. The body is 

 entirely destitute of limbs, and is enclosed in an integument 

 which is thrown into numerous transverse wrinkles, and some- 

 times has numerous horny scales imbedded in it. The eyes are 

 concealed by the skin, and are rudimentary. There is no tail, 

 and the anal aperture is placed almost at the extreme end of 

 the body. When adult, respiration is carried on by means of 

 lungs, but gills are present in the young, and there can, there- 

 fore, be no doubt as to their being genuine Amphibians. 



The CcecilicB are found burrowing in marshy ground, and 

 they are not unlike large earth-worms in appearance, but they 

 sometimes attain a length of several feet. 



