But not a single
characteristic of this section is retained by the brood of some Prawns
belonging to the genus Peneus or in its vicinity. These quit the egg
with an unsegmented ovate body, a median frontal eye, and three pairs of
natatory feet, of which the anterior are simple, and the other two
biramose--in fact, in the larval form, so common among the lower
Crustacea, to which O.F. Muller gave the name of Nauplius. No trace of a
carapace! no trace of the paired eyes! no trace of masticating organs
near the mouth which is overarched by a helmet-like hood!
(FIGURE 28. Nauplius of a Prawn, magnified 45 diam.
FIGURE 29. Young Zoea of the same Prawn, magnified 45 diam.
FIGURE 30. Older Zoea of the same Prawn, magnified 45 diam.
FIGURE 31. Mysis-form of the same Prawn, magnified 45 diam.)
In the case of one of these species the intermediate forms which lead
from the Nauplius to the Prawn, have been discovered in a nearly
continuous series.
The youngest Nauplius (Figure 28) is immediately followed by forms in
which a fold of skin runs across the back behind the third pair of feet,
and four pairs of stout processes (rudiments of new limbs) sprout forth
on the ventral surface. Within the third pair of feet, powerful
mandibles are developed.
In a subsequent moult the new limbs (maxillae, and anterior and
intermediate maxillipedes) come into action, and in this way the
Nauplius becomes a Zoea (Figure 29), agreeing perfectly with the Zoea of
the Crabs in the number of the appendages of the body, although very
different in form and mode of locomotion and even in many particulars of
internal structure.
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