therefore the points of attachment in the two groups indicate homologous
parts of the body. From the point of attachment in the Rhizocephala the
roots penetrate into the body of the host, whilst in the Cirripedia, the
cement-ducts issue from the same point. The roots are blind tubes,
ramified in different ways in different species. The cement-ducts in the
basis of the Balanidae likewise constitute a generally remarkably
complicated system of ramified tubes, with regard to the mode of
termination of which nothing certain has yet been made out. Individual
caecal branches are not unfrequently seen even in the vicinity of the
carina; and, at least in some species, in which the cement-ducts divide
into extremely numerous and fine branchlets, forming a network which
gradually becomes denser towards the circumference of the basis, these
seem nowhere to possess an orifice.

Now as to the question: How were Cirripedia converted by natural
selection into Rhizocephala?

A considerable number of existing Cirripedia settle exclusively or
chiefly upon living animals;--on Sponges, Corals, Mollusks, Cetaceans,
Turtles, Sea-Snakes, Sharks, Crustaceans, Sea Urchins, and even on
Acalephs. Dichelaspis Darwinii was found by Filippi in the branchial
cavity of Palinurus vulgaris, and I have met with another species of the
same genus in the branchial cavity of Lupea diacantha.

The same thing may have taken place in primitive times. The supposition
that certain Cirripedes might once upon a time have selected the soft
ventral surface of a Crab, Porcellana or Pagurus, for its
dwelling-place, has certainly nothing improbable about it. If then the
cement-ducts of such a Cirripede instead of merely spreading on the
surface, pierced or pushed before them the soft ventral skin and