This valuable little animal, with its tiny head, round, elongated
body, and many pairs of caterpillar-like legs, was until a few
decades ago regarded as an Annelid (like the earth-worm). It has,
in point of fact, the peculiar kidney-structures (nephridia) and
other features of the Annelid, but a closer study discovered in
it a character that separated it far from any worm-group. It was
found to breathe the air by means of tracheae (little tubes
running inward from the surface of the body), as the myriapods,
spiders, and insects do. It was, in other words, "a kind of
half-way animal between the Arthropods and the Annelids"
("Cambridge Natural History," iv, p. 5), a surviving kink in the
lost chain of the ancestry of the insect. Through millions of
years it has preserved a primitive frame that really belongs to
the Cambrian, if not an earlier, age. It is one of the most
instructive "living fossils" in the museum of nature.
Peripatus, as the little animal is called, points very clearly to
an Annelid ancestor of 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.
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