CHAPTER LV 



CLASS AMPHIBIA 



The name Amphibia indicates that these animals Uve at different 

 times in their hfe history in two environments — water and air. They 

 generally possess a soft skin which is kept moist by an abundant mucous 

 secretion. No exoskeletal structures are developed in any living forms, 

 except in the Apoda, but some extinct amphibians possessed a more 

 or less complete dermal armor made up of bony plates. Typically they 

 possess four jointed legs. Two nostrils are present which open directly 

 into the anterior part of the mouth cavity. Lungs appear in amphibians 

 for the first time, developing as outpocketings from the ventral side 

 of the pharynx. These have thin elastic walls the superficial area of 

 which is increased by folds; the recesses between these folds are known 

 as alveoli. There are renal-portal and hepatic-portal systems. The 

 kidney is a mesonephros and the urinary ducts open into a cloaca. The 

 eyes are usually supplied with lids. The middle ear appears, and in 

 many species of frogs a flat, circular tympanic membrane is to be seen 

 behind each eye. 



392. Classification. — The class Amphibia is usually divided into 

 three orders: 



1. Urodela {u ro de' la; G., oura, tail, and delos, visible). — Amphibians 

 with tails, including salamanders and newts. 



2. Salientia (sa li en' shI a ; L., salientis, leaping). — Tailless amphib- 

 ians, including frogs and toads. 



3. Apoda (ap' o da; G., a, without, and podos, foot). — The cecilians, 

 which are legless types. 



393. Urodela. — The tailed Amphibia are the typical forms. They 

 retain a tail throughout life; possess limbs of a primitive character set at 

 right angles to the body, the fore- and hind limbs being approximately 

 equal in size; and have teeth in both jaws. These forms show a gradual 

 transition from aquatic to terrestrial life and may be divided into two 

 groups. Those which retain their gills and are aquatic throughout life 

 are known collectively as perennihranchs; those which lose their gills 

 upon becoming adult and assume a terrestrial mode of life are known 

 as caducibranchs. 



Among the perennihranchs living in eastern United States is the 

 mud puppy, Necturus, which has three pairs of fringed external gills 

 and a gill cleft behind each (Fig. 247 A). As in all of the perennihranchs 



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