ints back to the worms as their ancestors, the
divergence having perhaps originated, as we have suggested, in the
Rotatoria.

While the Crustacea may have resulted from a series of prototypes
leading up from the Rotifers (Fig. 198), it is barely possible that one
of these creatures may have given rise to a form resulting in two series
of beings, one leading to the Leptus form, the other to the Nauplius.
For the true Annelides (Chaetopods) are too circumscribed and homogeneous
a group to allow us to look to them for the ancestral forms of insects.
But that the insects may have descended from some low worms is not
improbable when we reflect that the Syllis and allied genera of
Annelides bear appendages consisting of numerous joints; indeed, the
strange Dujardinia rotifers, figured by Quatrefages, in its general form
is remarkably like the larva of Chloeon. It has a quite distinct head,
bearing five long, slender, jointed antennae, and but eight or nine rings
to the body, which ends in two long, many jointed appendages exactly
like the tentacles. Quatrefages adds, that its movements are usually
slow, but "when it wishes to move more rapidly, it moves its body
alternately up and down with much vivacity, and shoots forwards by
bounds, so to speak, a little after the manner of the larvae of the
mosquito" (Histoire Naturelle des Anneles, Tome 2, p. 69). The gills of
aquatic insects only differ from those of worms in possessing tracheae,
though the gills of the Crustacea may be directly compared with those of
insects.

[Illustration: 198. A Rotifer.]

But when once inside the circle of the class of insects the ground is
firmer, as our knowledge is surer. Granting now that the Leptus-like
ancestor of the six-footed insects has become established, it is not so
difficult to see how the Podurae and finally a form like Campodea
appeared. Aquatic forms resembling the larva of the Ephemerae, Perlae and,
more remotely, the Forficulae and white ants of to-day were probably
evolved with comparative suddenness. Given the evolution of forms like
the earwigs (Forficula), cockroaches and white ants (Termes), the latter
of which abounded in the coal period, and it was not a great step
forward to the evolution of the Dragonflies, the Psocus, the Chrysopa,
the lice or parasitic Hemiptera, together with Thrips, thus forming the
establishment of lines of development leading up to those Neuroptera
with a complete metamorphosis, and finally to t