250 



VERTEBRATE ANIMALS. 



Toads, and in some others, there are two sets of gills, one external 

 and the other internal, of which the former is soonest lost. The 

 lungs of the Amphibians never attain a very high state of develop- 

 ment ; and in those forms in which the gills are retained throughout 

 life, the chief business of respiration appears to be cai'ried on by the 

 •gills. In accordance with the changes in the respiratory process, 

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

 develo2Mnent 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 proportionate diminu- 

 tion. At hrst 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 Amphihia are divided into three living and one extinct order 

 as follows ; — 



OrDER I. 0PHTOMORPHA. 



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 C'cecilice (fig. 180), which ai-e found in 



Vv^. 180. — a Siplwnopf; aivuilafuR^ one of the Ceecilians, much reduced ; h Head of the 

 sanii' ; c JInuth, showing the tongue, teeth, and internal openings of the nostrils ; 

 d Tail of the same. (After Dunieril and Bibron.) 



Southern Asia, Africa, and Sonth America. The body is entirely 

 destitute of limbs, and is enclosed in an integument which is tlirown 

 into numerous transverse wrinkles, and sometimes has numerous 



