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
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.
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