It may, however, be remarked, that the different curvature of the embryo
in the Amphipoda and Isopoda is so far instructive, as it proves that
their present mode of development was adopted only after the separation
of these orders, and that, in the primitive stock of the Edriophthalma,
the embryos were, if not Nauplii, at least short enough in the body to
find room in the egg in an extended position, like the larvae of
Achtheres enclosed by the Nauplius-skin. On the other hand the
uniformity of development that prevails in each of the two orders--which
is expressed in the Amphipoda for example in the formation of the
"micropylar apparatus," in the Isopoda in the want of the last pair of
ambulatory feet--testifies that the present mode of development has come
down from a very early period and extends back beyond the separation of
the present families. In these two orders also, as well as in the Crabs,
we can hardly hope to find traces of earlier young states, unless it be
in the family of the Tanaidae.* (* Whether the want of the abdominal
feet in the young of Tanais be an inheritance from the time of the
primitive Isopoda, or a subsequently acquired peculiarity, which appears
to me the more admissible view at present, may perhaps be decided with
some certainty, when we become acquainted with the development and mode
of life of its family allies, Apseudes and Rhoea.
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