INTRODUCTION. XV11 
part of these reptiles undergo a transition, as they advance in 
age, from a fish-like tadpole furnished with gills, to a four- 
legged animal with lungs. Others never lose their gills, 
though they acquire lungs, and of this kind are the siren and 
the proteus. 
THE PISCES, 
Or Fishes, are defined by Cuvier to be vertebrated animals with red 
blood, breathing through the medium of water by means of their 
branchiae or gills. To this definition may be added, that fishes have 
no neck, and that the body generally tapers from the head to the 
tail; that most of the species are furnished with air-bladders 
which enable them to swim ; and that their bodies are generally co- 
vered with scales. The heart has only one auricle and the blood is 
cold. The gills require to be kept moist to enable the fish to 
breathe, and as soon as the gills become dry, the fish dies. Thus 
fishes with large gills die almost as soon as they are taken out of 
the water ; while those with very small gills, like the eel, live a long 
time. Fishes have no feet, but they are furnished with fins. The 
scientific knowledge of Fishes is called Ichthyology. Fishes are 
first divided into two great series, viz., the Bony Fishes, and the 
Cartilaginous Fishes, and these are again subdivided into nine orders, 
as follows : 
OSSEOUS OR BONY FISHES. 
1. Acanthropterygii, or fishes with hard fins. 
2. Malacopterygii abdominales, or fishes with soft abdominal fins. 
3. Mulacopterygii sub-brachiati, or fishes with soft fins under the 
gills. 
4. Malacopterygii apodes, or fishes without ventral fins. 
5. Lopobranchii, or fishes with tufted branchiae. 
6. Plectognathii, or fishes with the upper jaw fixed. 
CHONDROPTERYGIC, OR CARTILAGINOUS FlSHES. 
7. Cyclostomi, or fishes with jaws fixed in an immoveable ring, and 
with holes for the gills. 
8. Selachii, or fishes with moveable jaws and holes for the gills. 
9 Sturiones, with the branchiae in the usual form. 
Of the bony fishes the Acanthropterygii, or fishes with hard spiny 
fins, are divided into fifteen families, the principal of which are the 
perch family, the mailed cheek fishes, including the gurnards, the fly- 
ing fish of the Mediterranean, and the sticklebacks, or jack banticles ; 
