We have seen that the arthropods were the first of all animals to conquer the realm of the air, the earliest insects appearing in the Ordovician.  Insects had now become exceedingly abundant, and the Carboniferous forests swarmed with the ancestral types of dragon flies,—­some with a spread of wing of more than two feet,—­ May flies, crickets, and locusts.  Cockroaches infested the swamps, and one hundred and thirty-three species of this ancient order have been discovered in the Carboniferous of North America.  The higher flower-loving insects are still absent; the reign of the flowering plants has not yet begun.  The Paleozoic insects were generalized types connecting the present orders.  Their fore wings were still membranous and delicately veined, and used in flying; they had not yet become thick, and useful only as wing covers, as in many of their descendants.

Fishes still held to the Devonian types, with the exception that the strange ostracoderms now had perished.

Amphibians.  The vertebrates had now followed the arthropods and the mollusks upon the land, and had evolved a higher type adapted to the new environment.  Amphibians—­the class to which frogs and salamanders belong—­now appear, with lungs for breathing air and with limbs for locomotion on the land.  Most of the Carboniferous amphibians were shaped like the salamander, with weak limbs adapted more for crawling than for carrying the body well above the ground.  Some legless, degenerate forms were snakelike in shape.

The earliest amphibians differ from those of to-day in a number of respects.  They were connecting types linking together fishes, from which they were descended, with reptiles, of which they were the ancestors.  They retained the evidence of their close relationship with the Devonian fishes in their cold blood, their gills and aquatic habit during their larval stage, their teeth with dentine infolded like those of the Devonian ganoids but still more intricately, and their biconcave vertebrae which never completely ossified.  These, the highest vertebrates of the time, had not yet advanced beyond the embryonic stage of the more or less cartilaginous skeleton and the persistent notochord.

On the other hand, the skull of the Carboniferous amphibians was made of close-set bony plates, like the skull of the reptile, rather than like that of the frog, with its open spaces (Figs. 313 and 314).  Unlike modern amphibians, with their slimy skin, the Carboniferous amphibians wore an armor of bony scales over the ventral surface and sometimes over the back as well.

It is interesting to notice from the footprints and skeletons of these earliest-known vertebrates of the land what was the primitive number of digits.  The Carboniferous amphibians had five-toed feet, the primitive type of foot, from which their descendants of higher orders, with a smaller number of digits, have diverged.

The Carboniferous was the age of lycopods and amphibians, as the Devonian had been the age of rhizocarps and fishes.