unite as the descendants of the same primitive ancestors, Mysis with the
Isopoda on the one hand, and on the other the rest of the Podophthalma
with the Amphipoda? I think not. Such a necessity exists only for those
who estimate a peculiarity at a higher value because it makes its
appearance at an earlier period of the egg-life. Whoever regards species
as not created independently and unchangeably, but as having gradually
become what they are, will say to himself that, when the ancestors of
our Mysides came (probably much later than those of the Amphipoda and
Isopoda) to develop numerous body-segments and limbs whilst still
embryos, as they could no longer find room in the egg when extended
straight out, and were therefore compelled to bend themselves, this
could only take place either upwards or downwards, and whatever
conditions may have decided the direction actually adopted, any near
relationship to either of the two orders of Edriophthalma could hardly
have taken part in it.

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