Full resolution (JPEG) - On this page / på denna sida - II. The development
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very small cylindrical prominence. In all the specimens seen the body inside the
cuticle had contracted and left a space in which always some characteristic small,
drop-like, deeply stained bodies were found; in moulting they are left in the cast
cuticle.
The next stage c) is a second Metanauplius, ca. 0,23—0,24mm in length (Fig.
16—17); the appendages of the former stage are preserved, and in addition maxillæ (mx),
two anterior pairs of swimming feet (p1, p2) and a rudiment of the posterior (p3) have
appeared. The maxillulæ are as in the first Metanauplius, only the “shaft” a little larger
and — as seen in sections — provided with extremely feeble muscles. The maxillæ
are large, directed backwards, as are the swimming feet. The first and second pair
of the latter show distinctly a basal part and two clumsy branches, the outer
considerably larger than the inner and provided with a few short bristles. The third and
fourth pairs are simple conical rudiments. The swimming feet are set wider apart
from each other than the maxillæ, and the thoracic segments, especially the anterior
are distinguishable. The central nervous system forms a dark mass easily seen in
profile against the transparent back; in sections the composing ganglionic elements are
quite distinct. Muscles to the fully developed appendages as well as rudiments of
the longitudinal body-muscles and of those of the developing limbs are observable.
The body is somewhat pear-shaped, the back arched, the ventral side more flat but
with a prominent mouthpart, now carrying an opening. In sections the rudiment of
an oesophagus is seen, passing between the brain and the foremost ventral ganglia
and abutting on the embryonic entoderm, not yet forming a hollow stomach. An
antennal gland is distinct, also a shell-gland (maxillary-gland). Thus, both these
glands here are found together, while generally in Copepoda the antennal gland is
said to occur in the larva, and the shell-gland to appear later.
The next moulting produces d) a third Metanauplius (Fig. 18—19). The size has
not altered much, the length being ca. 0,248—0,280 mm, and the shape of the body
is almost the same, only the mouth is less prominent, and the segmentation more
distinctly marked, corresponding to the greater development of the thoracic limbs.
Most interesting are the appendages. The outer ramus of the antennæ (a2) at first
sight seems lost; closer examination shows, that on its place is an empty cuticular
case, without any setæ, while the inner ramus has the same shape as in former stages.
The mandibles (md) are still more reduced: both rami and almost the whole shaft
form empty and naked sheaths, only at the very base of the shaft enclosing a minute
papilla of tissue. The maxillulæ — from the first appearance rather rudimentary —
are now completely lost. The maxillæ (mx), on the other hand, are further developed,
large and removed from the swimming feet, and directed forwards; they have now
distinctly three segments, the terminal one rounded and carrying a hooked spine.
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