_Cirripedia having a carapace, consisting either of a capitulum on a
peduncle, or of an operculated shell with a basis. Body formed of six
thoracic segments, generally furnished with six pairs of cirri; abdomen
rudimentary, but often bearing caudal appendages; mouth with the labrum
not capable of independent movements; larva firstly uniocular, with
three pairs of legs, lastly, binocular, with six pairs of thoracic
legs._
In the sketch of the three Orders given in the Introduction, it will
have been seen that the differences in their structure are so great,
that it would have been hardly possible to have given a single blended
account of the whole Class. But as all common Cirripedes are included
in the present Order, here would have been the natural place for a
full description of their external and internal structure. Having,
however, been necessarily, yet perhaps unfortunately, led to give, in
my former volume, a description of this kind of the Lepadidæ; and as it
is necessary to give a similar account of the other great family of the
Order, namely, the Balanidæ, I have found it more convenient to make
this latter account comparative and supplemental to the former one on
the Lepadidæ, and so serve for the Order, rather than attempt to give
a separate description in full of it. For this latter plan would have
involved much useless repetition, as, on account of the many exceptions
and limitations necessary to almost every statement, there is little
choice between a description of great length and a mere diagnostic
character of the Order, such as I have given above.
The Thoracica may be divided into three very natural Families, of
nearly equal value; firstly, the Balanidæ, or sessile Cirripedes,
which may be subdivided into two sub-families, also very natural, the
Balaninæ and Chthamalinæ; secondly, the Verrucidæ, containing only
one genus; and thirdly, the Lepadidæ, or pedunculated Cirripedes.
These three families differ from each other, besides in mere external
appearance, almost exclusively in the relation of the different
portions of their external covering or carapace, and of the muscles
moving such portions. In the Balanidæ, the four opercular valves
surrounding the orifice leading into the sack, are capable of other
movements, besides being opened and shut; whereas all the other valves
are immoveably united together. In the Lepadidæ, the valves answering
to the opercular valves, are furnished with a muscle only for shutting
them; whereas the peduncle answering to the basis in the Balanidæ is
capable of various movements. In the Verrucidæ the shell is singularly
asymmetrical; only half of the operculum (either the right or the left
side, this varying even in the same species) being moveable; the other
half being immoveably united to the remaining valves; and the whole
shell has only one muscle serving to shut the moveable half of the
operculum. All the internal parts and organs are very similar in the
above three Families. If, however, the internal structure of one of the
two sub-families, into which the Balanidæ may be divided, namely, of
the Balaninæ, be compared with that of the Lepadidæ, several important
differences may be detected;--on the one hand, in the Balaninæ, the
presence of branchiæ, the extremely complicated cementing apparatus,
the difference in structure between the third and succeeding pairs of
cirri, the large palpi, the notched labrum, and the laterally double
teeth of the mandibles;--and on the other hand, in the Lepadidæ, the
presence of ovigerous fræna, caudal appendages, bullate labrum, and
often prominent olfactory orifices. But if the Lepadidæ be compared in
these several respects with the other sub-family, or Chthamalinæ, which
cannot possibly be removed out of the family of Balanidæ, many of these
differences break down and disappear, in some or all of the species.
The Lepadidæ include, as has previously been noticed, a much greater
range of difference than the Balanidæ; and this is what might have
been expected, for it is the most ancient family, and extinction has
done its work, separating genera, which, in accordance to analogy, we
may suppose were once more nearly connected by intermediate forms. The
Lepadidæ, in one sense, may be taken as the type of their order; for
they have undergone less "morphological differentiation;" that is, they
differ the least from the last larval stage, and seem to give the most
_general_ idea of a Thoracic Cirripede. On the other hand, if we mean,
as some authors do, by the word type, that form which, in the group in
question, has been most modified, and illustrates every peculiarity of
its class in the strongest manner, then we must look to the Balaninæ,
and to its typical genus, Balanus, for the most Cirripedial form. In
this genus the different portions of the carapace differ most, and
subserve to a certain extent different ends, and in minute structure
are most complicated; here the cementing apparatus, which offers the
main characteristic of the whole sub-class, is most complex; here the
several pairs of cirri differ most from each other in structure and
action; here the peculiar branchiæ (organs apparently derived from the
modification of another organ, itself confined to Cirripedes, viz., the
ovigerous fræna) are best developed; here the nervous system is most
highly concentrated; and, lastly, here we meet with the largest and
most massive species of the whole group.