This applies also
to the young males of the Shore-hoppers (Orchestia) with regard to the
second pair of anterior feet (gnathopoda). In like manner no one will
hesitate to accept the possession of seven pairs of feet as a "typical"
peculiarity of the Edriophthalma, which Agassiz, on this account, names
Tetradecapoda; the young Isopoda, which are Dodecapoda, are also in this
respect further from the "type" than the adults.
It is certainly a rule, and this Darwin's theory would lead us to
expect, that in the progress of development those forms which are at
first similar gradually depart further from each other; but here, as in
other classes, the exceptions, for which the Old School has no
explanation, are numerous. Not unfrequently we might indeed directly
reverse the proposition and assert that the difference becomes the
greater, the further we go back in the development, and this not only in
those cases in which one of two nearly allied species is directly
developed, and the other passes through several larval stages, such as
the common Crayfish and the Prawns which are produced from
Nauplius-brood. The same may be said, for example, of the Isopoda and
Amphipoda. In the adult animals the number of limbs is the same; at the
first sight of a Cyrtophium or a Dulichia, and even after the careful
examination of a Tanais, we may be in doubt whether we have an Isopod or
an Amphipod before us; in the newly-hatched young the number of limbs is
different, and if we go back to their existence in the egg, the most
passing glance to see whether the curvature is upwards or downwards
suffices to distinguish even the youngest embryos of the two orders.
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