We saw that a vast
and varied wormlike population must have filled the Archaean
ocean, and that all the higher lines of animal development start
from one or other point in this broad kingdom. The Annelids, in
which the body consists of a long series of connected rings or
segments, as in the earth-worm, are one of the highest groups of
these worm-like creatures, and some branch of them developed a
pair of feet (as in the caterpillar) on each segment of the body
and a tough, chitinous coat. Thus arose the early Arthropods, on
tough-coated, jointed, articulated animals. Some of these
remained in the water, breathing by means of gills, and became
the Crustacea. Some, however, migrated to the land and developed
what we may almost call "lungs"--little tubes entering the body
at the skin and branching internally, to bring the air into
contact with the blood, the tracheae.
In Peripatus we have a strange survivor of these primitive
Annelid-Tracheates of many million years ago. The simple nature
of its breathing apparatus suggests that the trachere were
developed out of glands in the skin; just as the fish, when it
came on land, probably developed lungs from its swimming
bladders.
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