222 TlMEHRI. 
this type is confined entirely to Mexico : species of 
the commonly known " two-headed snake" (Atnphis- 
bdena) are also to be seen ; in this type — a true 
lizard and not a snake — the limbs are lost, the body 
becoming serpentiform ; the common name has been 
derived from the fa6l that each end of the body is about 
equally thickened, the tail being not easily distinguished 
from the head. Further reference will be made to the 
reptiles in connexion with the case of stuffed specimens. 
Close to these reptiles in spirit, on the right, are shewn 
specimens of colonial Amphibia, the group of the frogs 
and toads. These animals were once classed with the 
reptiles, but are now separated, being more closely allied 
to the fishes, as is shown by the presence of gills during 
the early part, and sometimes during the whole, of life. 
The essential chara6leristic of the animals of this group 
is the marked metamorphosis that they undergo — a series 
of changes that is well exemplified by specimens exhibited 
of the paradoxical frog or " frog-fish" (Pseudis para- 
doxus). The changes may be thus briefly described. 
From the egg of the frog or toad, as the case may be, is 
hatched a curious elongated fish-like form, known as the 
tadpole. The tadpole does not breathe by means of 
lungs, as do the adult frogs, but is furnished with 
external and internal gills. The external gills soon 
fall, and the tadpole then breathing air in the water by 
means of internal gills is perfe6lly fish-like. The head 
at this stage is very large, and is separated from the 
body by a long slit which, as in fishes, leads to the gill 
chamber ; while the heart is also like a fish's heart, 
driving blood to the gills to be purified. As the tadpole 
grows, however, true lungs begin to appear, and the gills 
