ORDER OF CETACEA. 
The Cetacea are essentially aquatic animals, externally resem- 
bling Fishes, but belonging really, by tbeir whole structure, to the 
class of Mammalia. They would indeed be strange Fishes, these 
creatures, which have mammae with which to suckle their young, 
which breathe not by gills, but by lungs, and which have a heart 
provided with two ventricles and two auricles. 
The Cetacea, then, are Mammalia. Only, instead of being 
organized for living on land, they are admirably suited for the 
water ; some of them acquire enormous dimensions, and are the 
Igiants of the animal kingdom. 
Their body, more or less spindle-shaped, is terminated behind in 
a tail, which becomes so broad as to form a fin ; this fin is trans- 
versal, not vertical as in fishes. The tail is the principal agent in 
moving these living masses. 
On the back of most of the Cetacea there exists another fin, 
which is merely a modification of the skin. 
The Cetacea have no posterior members. Their anterior 
members are transformed into natatory paddles, which are of com- 
paratively little use for locomotion through the water, and of 
which the principal use, no doubt, is to balance their movements. 
These anterior members, thus changed into flippers, present, essen- 
tially, the same structure as does the corresponding member in 
other Mammalia — the paw of the Dog, the wing of the Bat, &c. 
Their nostrils open, in general, from the upper part of the head. 
Owing to this position of the nostrils, which are placed higher 
than the mouth, these animals can breathe the air without coming 
too much out of the water. These same organs play still another 
part, which we will describe presently. 
The skin of Cetacea is generally quite hairless, which very rarely 
happens in the case of other Mammalia. Their teeth are mostly 
conical, uniform, and sometimes numerous. All their tissues, but 
especially their sub-cutaneous cellular tissue, are impregnated with 
