In one Cypridina I find branchiae of considerable size, which are
entirely wanting in another species, but this does not appear to me to
be a reason for separating these species even generically.
On the other hand, what do we know of the physiological significance of
the number of segments, and all the other matters which we are
accustomed to regard as typical peculiarities of the different organs,
and to which we usually ascribe the highest systematic value?
"Those peculiarities which first appear, should be more highly estimated
than those which appear subsequently. A system, in order to be true and
natural, must agree with the sequence of the organs in the development
of the embryo." If the earlier manifested peculiarities are to be
estimated more highly than those which afterwards make their appearance,
then in those cases in which the structure of the adult animal requires
one position in the system, and that of the larva another, the latter
and not the former must decide the point. As the Lernaeae and
Cirripedes, on account of their Nauplius-brood, were separated from
their previous connexions and referred to the Crustacea, we shall, for
the same reason, have to separate Peneus from the Prawns and unite it
with the Copepoda and Cirripedia. But the most zealous embryomaniac
would probably shrink from this course.
A "true and natural system" of the Crustacea to be in accordance with
the sequence of the phenomena would have to take into account in the
first place the various modes of segmentation, then the position of the
embryo, next the number of limbs produced within the egg and so forth,
and might be represented somewhat as follows:--
CLASSIS CRUSTACEA.
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