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Not only by their structure and embryology, as well as their metamorphosis, do the myriopods and insects stand apart from the Arachnida and other arthropods, but it seems probable that they have had a different ancestry, the arthropods being apparently polyphyletic.

There are two animals which appear to connect the insects with the worms, and which indicate a separate line of descent from the worms independent of that of the other classes. These are the singular Peripatus, which serves as a connecting link between arthropods and worms, and Scolopendrella (Symphyla). These two animals are guide-posts, pointing out, though vaguely to be sure, the way probably trod by the forms, now extinct, which led up to the insects.

Relations of Peripatus to Insects.

The tracheæ (Fig. 4, D, tr) are unbranched fine tubes, without a “spiral thread,” and are arranged in tufts, in P. edwardsii opening by simple orifices or pores (“stigmata”) scattered irregularly over the surface of the body; but in another species (P. capensis) some of the stigmata are arranged more definitely in longitudinal rows,—on each side two, one dorsally and one ventrally. “The stigmata in a longitudinal row are, however, more numerous than the pairs of legs.” (Lang.)