processes in the development of the Crustacea. Until recently it was
regarded as a general rule that, by the partial segmentation of the
vitellus a germinal disc was formed, and in this, corresponding to the
ventral surface of the embryo, a primitive band. We now know that in the
Copepoda (Claus), in the Rhizocephala (Figure 64), and, as I can add, in
the Cirripedia (Figures 61 to 63) the segmentation is complete, and the
embryos are sketched out in their complete form without any preceding
primitive band. Probably the latter will always be the case where the
young are hatched as true Nauplii (and not merely with a Nauplius-skin,
as in Achtheres). The two modes of development may occur in very closely
allied animals, as is proved by Achtheres among the Copepoda.* (* I have
not mentioned the Pycnogonidae, because I do not regard them as
Crustacea; nor the Xiphosura and Trilobites, because, having never
investigated them myself, I knew too little about them, and especially
because I am unacquainted with the details of the explanations given by
Barrande of the development of the latter. According to Mr. Spence Bate
"the young of Trilobites are of the Nauplius-form.")


CHAPTER 10. ON THE PRINCIPLES OF CLASSIFICATION.

Perhaps some one else, more fortunate than myself, may be able, even
without Darwin, to find the guiding clue through the confusion of
developmental forms, now so totally different in the nearest allies, now
so surprisingly similar in members of the most distant groups, which we
have just cursorily reviewed. Perhaps a sharper eye may be able, with
Agassiz, to make out "the plan established from the beginning by the
Creator,"* (* "A plan fully matured in the beginning and undeviatingly
pursued;" or "In the beginning His plan was formed and from it He has
never swerved in any particular" (Agassiz and Gould, 'Principles of