ICHTHYOPSIDA. 247 
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CHAPTER XXVI. 
CLASS II. AMPHIBIA. 
THIS class of Vertebrata comprises the Frogs and Toads, the 
Newts and Land-salamanders, the Ceci/ieg, and some extinct 
forms, and it may be briefly defined as follows: In all cases 
gills or branchize adapted for aquatic respiration are present 
during a part or the whole of life; but in all cases true lungs 
adapted for breathing air are ultimately developed, even when 
the gills are retained through life. All pass through some sort 
of a metamorphosis after being set free from the egg. The 
limbs may be absent or there may be only one pair, but in no 
case are they ever converted into fins. When median fins 
are present, as is sometimes the case, these are never furnished 
with fin-rays or interspinous bones, as in the fishes. The skull 
always articulates with, or is jointed to, the spinal column by 
zwo articular surfaces or condyles. The heart consists of two 
auricles and a single ventricle. The nasal sacs always open be- 
hind into the mouth; and there is a common cavity or “ cloaca” 
which receives not only the termination of the intestine (vectumz), 
but also the ducts of the kidneys and of the reproductive organs. 
The great and distinguishing character of the Amphzbza is 
that they invariably undergo some kind of metamorphosis after 
birth ; though in some rare cases the eggs are retained so long 
within the body of the parent that there is little or no obvious 
change. In the great majority of cases, however, the Amphibians 
commence life as water-breathing larve, provided with gills ; 
but in their adult state they possess true air-breathing lungs, the 
gills sometimes disappearing when the lungs are developed, but 
being sometimes retained throughout life. In the most of 
cases the gills are 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 
