NOTES. 3(3 
7° In the human lungs, they number 600,000,000, each about 45 of an inch 
in diameter, with an aggregate area of 132 square feet. The thickness of the 
membrane between the blood and the air is g@5p5 of aninch. The lungs of 
Carnivores are more highly developed than those of Herbivores. In the 
Manatee, they are not confined to the thorax, but extend down nearly to 
the tail. 
77 Crocodiles are the only Reptiles whose nostrils open in the throat be- 
hind the palate, instead of directly into the mouth-cavity. This enables the 
Crocodile to drown its victim without drowning itself; for, by keeping its 
snout above water, it can breathe while its mouth is wide open. 
78 A rudimentary diaphragm is seen in the Crocodile and Ostrich. 
7 The poison-glands of venomous Serpents and the silk-vessels of Cater- 
pillars are considered to be modified salivary glands. Birds, Snakes, and 
Cartilaginous Fishes have no urinary bladder. 
8° Since the weight of a full-grown animal remains nearly uniform, it must 
lose as much as it receives; that is, the excretions, including the solid re- 
siduum ejected from the intestinal canal, equal the food and drink. 
81 Other names for derm are, cutis, corium, enderon, and true skin; and for 
epidermis, ewticle, ecderon, and scarf-skin. The derm is often so intimately 
blended with the muscles, that its existence as a distinct layer is not easily 
made out. Even in Infusoria, we find the tunic double, an outside cutiewa 
lined by a soft cortical layer; and in Jelly-fishes, naturalists distinguish an 
ectoderm and endoderm. 
52 See Fig. 146. Papillee are scarcely visible in the skin of Reptiles and 
Birds. 
53 The animal basis of this structure is chitine, a peculiar substance found 
in the hard parts of all the articulated animals. 
84 The large claws within the old crust are soft, and hence are able to be 
drawn through the small joints. 
85 The shell is always an epidermal structure, eyen when apparently inter- 
nal. The horny ‘‘pen”’ of the Squid, the ‘‘bone”’ of the Cuttle-fish, and the 
caleareous spot on the back of the Slug, are only concealed under a fold of 
the mantle. So the shell of the common Unio, or Fresh-water Clam, is coy- 
ered with a brownish or greenish membrane, which is the outer layer of the 
epidermis. Where the mantle covers the lips of a shell, as in most of the 
large sea-snails, or where its folds cover the whole exterior, as in the pol- 
ished Cowry, the epidermis is wanting, or covered up by an additional layer. 
86 The pearls of commerce, found in the mantle of some Mollusks, are simi- 
lar in structure to the shell; but what is the innermost layer in the shell, is 
placed on the outside in the pearl, and is much finer and more compact. The 
pearl is formed around some nucleus, as an organic particle, or grain of sand. 
57 When the centrum is concave on both sides, as in Fishes, it is said to be 
aniphicelous ; when concave in front and convex behind, as in Crocodiles, it 
is called procelous ; when concave behind and convex in front, as in the neck- 
vertebre of the Ox, it is opisthocelous. In the last two cases, the vertebrae 
unite by ball-and-socket joints. 
88 A few have but one pair, the Whale and Siren wanting the hind pair; 
while some have none at all, as the Snakes and lowest Fishes. In land ani- 
