526 AMPHIBIA. 



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ency, the Reptiles have a skin which is capable of secreting a 

 uid by which it is preserved in a humid condition. The healthy 

 action of the skin, co-operating with that of the lungs, is really 

 essential to their existence. Dr. Townson, of England, in his 

 tracts on the " Respiration of the Amphibia," states from actual 

 experiment, that a frog, when placed on blotting paper well 

 soaked with water, absorbed nearly its own weight of the fluid 

 in the short time of an hour and a half; and, it is believed, these 

 reptiles never discharge it, except when they are disturbed or 

 pursued, and then only to lighten their bodies and facilitate their 

 escape. The form of the Amphibians is variable. Besides the 

 naked skin, we may mention, as general characters of the pres- 

 ent order, that the skull is united to the column of vertebrae by 

 two condyles, (Gr. kondulos, a protuberance or knot on the end 

 of a bone,) situated on the back of the head ; the teeth are gen- 

 erally numerous, of small and equal size and close set ; the toes 

 usually unfurnished with claws ; the ribs either wanting, (Plate 

 XII. fig. 4,) or rudimentary and not attached to the breast-bone ; 

 and the animals are oviparous, the eggs having soft, not calcare- 

 ous shells. A change of form and habit, as we have stated in 

 our general description of the Reptiles, occurs in many, which 

 begin their existence with branchiae, or gills, that afterwards be- 

 come obliterated ; while in others, the branchiae continue through- 

 out their lives. Several prominent naturalists, including Prof. 

 Agassiz, are inclined to separate the Amphibians from the Class 

 of Reptiles, regarding them as possessing the distinguishing 

 characters upon which classes are founded ; but as this point 

 seems not perfectly settled, and they have usually been numbered 

 with the Reptiles, they are so arranged on the Chart. 



The Amphibians may be divided into two sub-orders. 



FIRST SUB-ORDER. CADUCIBRANCHIATA, (Lat. caducus, perish- 

 able ; branchia, gills.) 



The distinguishing characteristic of this sub-order is that the 

 Amphibia which it includes, commence life with gills for the 

 aeration of the blood, i. e., the air effects, through the medium 

 of the gills, a change corresponding to the arterialization of the 

 blood through the medium of the lungs, in other animals. The 

 gills, however, are possessed only in the early or tadpole state ; 

 they become gradually obliterated, and lungs are developed. 



This sub-order includes five families, viz.: (1) CcBciliida, 

 Csecilia ; (2) Ranidce, Frogs ; (3) Bufoidcz, Toads ; (4) Salaman- 

 drida, Salamanders ; (5) Amphiumidce, Menopoma, &c. 



