penetrated into the interior of the host, this must have been beneficial
to the animal, because it would be thereby more securely attached and
protected from being thrown off during the moulting of its host.
Variations in this direction were preserved as advantageous.

But as soon as the cement-ducts penetrated into the body-cavity of the
host and were bathed by its fluids, an endosmotic interchange must
necessarily have been set up between the materials dissolved in these
fluids and in the contents of the cement-ducts, and this interchange
could not be without influence upon the nourishment of the parasite. The
new source of nourishment opened up in this manner was, as constantly
flowing, more certain than that offered by the nourishment accidentally
whirled into the mouth of the sedentary animal. The individuals favoured
in the development of the cement-ducts now converted into nutriferous
roots, had more than others the prospect of abundant food, of vigorous
growth, and of producing a numerous progeny. With the further
development, assisted by natural selection, of the roots embracing the
intestine of the host and spreading amongst its hepatic tubes, the
introduction of nourishment through the mouth and all the parts
implicated in it, such as the whirling cirri, the buccal organs, and the
intestine, gradually lost their importance, became aborted by disuse,
and finally disappeared without leaving a trace of their existence.
Protected by the abdomen of the Crab, or by the shell inhabited by the
Pagurus, the parasite also no longer required the calcareous test, in
which, no doubt, the first Cirripedes settling upon these Decapods
rejoiced. This protective covering, having become superfluous, also
disappeared, and there remained at last only a soft sack filled with
eggs, without limbs, without mouth or alimentary canal, and nourished,
like a plant, by means of roots, which it pushed into the body of its
host. The Cirripede had become a Rhizocephalon.