248 VERTEBRATE ANIMALS. 
which the former 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 
accordance with the changes in the respiratory process, corre- 
sponding 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, whilst the branchial 
vessels, which carry the blood to the gills, undergo a propor- ° 
tionate diminution. At first the condition of the circulation 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 extinct 
order, as follows :— 
1, Ophiomorpha. 
2. Urodela. 
3. Anoura. 
4. Labyrinthodontia. 
ORDER I. OPHIOMORPHA (Gr. ofhizs, a serpent ; and morphe, 
form).—This order is an extremely small one, and, as its name 
Fig. 133.—a Siphonops annulatus, one of the Cecilians, much reduced ; 6 Head of 
the same ; c Mouth, showing the tongue, teeth, anc internal openings of the nos- 
_trils; d Tail of the same. (After Dumeril and Bibron.) 
implies, it comprises certain snake-like Amphibians. The order 
includes only the curious animals known as Cecdlé@ (fig. 133); 
