the dentition, formation of the fins, number of vertebrae, etc.

And, to return to our Class of the Crustacea, has any particular
attention been paid in their classification to the distinctions
prevailing in the "most essential organs"? For instance, to the nervous
system? In the Corycaeidae, Claus found all the ventral ganglia fused
together into a single broad mass, and in the Calanidae a long ventral
chain of ganglia,--the former, therefore, in this respect resembling the
Spider Crabs and the latter the Lobster; but no one would dream on this
account of supposing that there was a relationship between the
Corycaeidae and the Crabs, or the Calanidae and the Lobsters.--Or to the
organs of circulation? We have among the Copepoda, the Cyclopidae and
Corycaeidae without a heart, side by side with the Calanidae and
Pontellidae with a heart. And in the same way among the Ostracoda, the
Cypridinae, which I find possess a heart, place themselves side by side
with Cypris and Cythere which have no such organ.--Or to the respiratory
apparatus? Milne-Edwards did this when he separated Mysis and Leucifer
from the Decapoda, but he himself afterwards saw that this was an error.
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