all the Tracheates (the myriapods, spiders, and
insects), or all the animals that breathe by means of trachere. To
understand its significance we must glance once more at an early chapter
in the story of life. 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. The primitive Tracheates,
delivered from the increasing carnivores of the waters, grew into
a large and varied family, as all such new types do in favourable
surroundings. From them in the course of time were evolved the three
great classes of the Myriapods (millipedes and centipedes), the
Arachnids (scorpions, spiders, and mites), and the Insects. I will
not enter into the much-disputed and Obscure question of their nearer
relationship. Some derive the Insects from the Myriapods, some the
Myriapods from the Insects, and some think they evolved independently;
while the rise of the spiders and scorpions is even more obscure.

But how can we see any trace of an Annelid ancestor in the vastly
different frames of these animals which are said to descend from it? It
is not so difficult as it seems to be at first sight. In the Myriapod
we still have the elongated body and successive pairs of legs. In
the Arachnid the legs are reduced in number and lengthened, while the
various segments of the body are fused in two distinct body-halves, the
thorax and t