Trans. N. Y. Ac. Scz. 166 May 8, 
sphere, flight requires special apparatus and structure and very great 
muscular power. But insects and birds outstrip all other creatures. 
Their whole structure is wonderfully adapted to aerial life ; the skele- 
ton light, the body filled with air, the wings elastic and light, but firm, 
and the wing muscles exceedingly powerful. Wings of birds are quite 
similar in principle and structure, but wings of insects greatly differ. 
Three classes of insect wings may be noticed: the transparent wings 
of the bee, fly, etc.: the dense opaque wing covers of the beetles : and 
the scale-covered wings of the butterfly and moth. 
The wing of the bat is an expansion of skin stretched upon the long 
fingers and from arm to leg. Dragons passed away with the age of 
reptiles. The so-called flying dragon has only a parachute of expanded 
skin, by aid of which it can sail downward through the air. 
The flying squirrel has a similar apparatus. In the flying- 
fish, the pectoral fins are very large, and serve a similar purpose. 
Some spiders are able to sail the air by means of kites or sails made of 
their silken webs. 
Locomotion on Solids —A much greater variety of organs are used 
for moving on solids. The leech, star-fish and cuttlefish employ suckers, 
which are little cupping glasses or diminutive water-pumps. The 
earth-wora has minute spines, and the serpent large scales or scutes on 
its belly ; caterpillars have pins rather than legs. The foot of the clam 
is a fleshy protuberance which can be pushed into the sand, enlarged at 
the end and then shortened. The snail glides over the surface by sets 
of short muscles in the under side of its body; flies adhere by a fluid, 
exuded by the hairs on the surface of their foot discs. 
. Of jointed limbs the millipede has, in some species, two or three 
hundred. The centipede has thirty or forty legs; crabs and lobsters 
have ten legs, but two used as pincers. The spider has eight legs, and 
the true insects have six legs. The reduction in the number of similar 
organs indicates advance in locomotive powers. 
As the skeletons of all vertebrates are built on the same general 
plan, the four limbs of all show a certain resem blance ; of course, the 
similarity is most evident in the legs of quadrupeds. 
The limb bones of fishes are in nearly all cases within the body, as 
the basis of the paired fins. But in some fishes they are sufficiently de- 
veloped to be useful for crawling. In reptiles the limbs are generally 
awkward on land, although some lizards are agile. The legs of birds 
are highly specialized. 
While mostly intended for running on land, the mammals neverthe- 
less exhibit a greater diversity of organs and methods of locomotion 
than any other class of animals. Here we find the swimming whale 
and seal, the flying bat, the sailing lemur and squirrel,{the creeping 
