Malacostraca, that is to say, are derived by inheritance from the same
source with them.

It would be easy to weave together the separate threads furnished by the
young forms of the various Crustacea, into a general picture of the
primitive history of this class. Such a picture, drawn with a little
skill, and finished in lively colours, would certainly be more
attractive than the dry discussions which I have tacked on to the
developmental history of these animals. But the mode of weaving in the
loose threads would still in many cases be arbitrary, and to be effected
with equal justice in various ways; and many gaps would still have to be
filled up by means of more or less bold assumptions. Those who have not
wandered much in this region of research would then readily believe that
they were standing upon firm ground, where mere fancy had thrown an airy
bridge; those acquainted with the subject, on the other hand, would soon
find out these weak points in the structure, but would then be easily
led to regard even what was founded upon well considered facts, as
merely floating in the air. To obviate these misconceptions of its true
contents from either side, it would be necessary to accompany such a
picture throughout with lengthy, dry explanations. This has deterred me
from further filling in the outline which I had already sketched.

I will only give, as an example, the probable history of the production
of a single group of Crustacea, and indeed of the most abnormal of all,
the RHIZOCEPHALA, which in the sexually mature state differ so
enormously even from their nearest allies, the Cirripedia, and from
their peculiar mode of nourishment stand quite alone in the entire
animal kingdom.

I must preface this with a few words upon the homology of the roots of