the Hermit Crabs, Pinnotheres, etc.,--and secondly and especially
because the peculiarities which distinguish the Zoea of this species
particularly from other Zoeae (the employment of the anterior limbs for
swimming, the furcate tail, the simple heart, the deficiency of the
paired eyes and abdomen at first, etc.) are neither to be deduced from a
retro-transfer of late-acquired advantages to this early period of life,
nor to be regarded at all as advantages over other Zoeae which the larva
might have acquired in the struggle for existence.

A similar development must have been once passed through by the
primitive ancestor of all Malacostraca, probably differing from that of
our Prawn, especially in the circumstance that it would go on more
uniformly without the sudden change of form and mode of locomotion
produced in the latter by the simultaneous sprouting forth and entering
into action in the Nauplius of four and in the Zoea of five pairs of
limbs. It is to be supposed that, not only originally but even still, in
the larvae of the first Malacostraca, the new body-segments and pairs of
limbs are formed singly,--first of all the segments of the fore-body,
then those of the abdomen, and finally those of the middle-body,--and,
moreover, that in each region of the body the anterior segments were
formed earlier than the posterior ones, and therefore last of all the
hindermost segment of the middle-body. Of this original mode more or
less distinct traces still remain, even in species in which, in other
respects, the course of development of their ancestors is already nearly
effaced. Thus the abdominal feet of the Prawn-larva represented in
Figure 33, are formed singly from before backwards, and after these the
last feet of the middle-body; thus, in Palinurus, the last two pairs of
feet of the middle-body are formed later than the rest; thus in the
young larvae of the Stomapoda the last three abdominal segments are
destitute of limbs, which are still wanting on the last of them in older