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The three anterior pairs of swimming feet are bifurcate, the outer ramus with short
bristles; the fourth pair is still undivided and small The ventral crest joining the
members of each pair of swimming feet in the Copepoda is already distinctly seen.
Antennal-and shell-glands are present as before, the latter now the greater.
e). The following, fifth, stage (Figs. 20, 21), is 0,288-0,312 mm in length; it is
to be called a Cyclopid (or Copepodid), as the shape now resembles that of a
Cyclops in so far, as the abdomen is fully developed, consisting of three distinct
segments, and all the swimming feet are biramous, although those of the last pair still
are smaller. The antennules (a1) often are held laterally outstretched, the antennæ
(a2) pointing forwards. The latter now are slender, unbranched, the last trace of the
outer branch having fully disappeared; they are indistinctly three-jointed, the last
joint with two terminal setæ. The mandibles are completely lost. The maxillæ are
strong, three-segmented, with claw-shaped terminal joint. The anterior three pairs
of swimming feet have the outer ramus longer and narrower than in the preceding
stage, and the setæ better developed; also the inner branch has grown but is much
smaller than the outer, and its setæ are quite small. Fourth pair is short and clumsy,
directed backwards and upwards, the rami broad and rounded, the setæ very small.
The segmentation of the body and the short tail is distinct; the telson of the latter
is flat, and carries in most specimens the two caudal setæ found in all the preceding
stages; in some of the oldest specimens two additional small setæ may be seen. The
intestine is filled with a large oil-drop, and ends blindly in the thorax without reaching
the abdomen. The ventral nervous system now does not show any distinct ganglia;
it is condensed into a mass, broad behind the mouth, pointed backwards between the
first pair of thoracic feet. Antennal-and shell-gland are as in the preceding stage.
The cyclopid stage is the last stage of metamorphosis which I have found enclosed
in the maternal gall; in some specimens, however, I have seen inside the cuticle of
its antennules a new antennule resembling very much that of the larval stage, found
outside the gall; it seems therefore pretty certain, that the Cyclopid will change into
the sixth stage, f) the Cyclops-larva (Figs. 22—24). This stage has been found
either attached to the walls of the bursal pouches in Asteronyx or loosely imbedded
in the connective tissue of these walls. Probably the moulting, which produces this
larva, takes place inside the maternal gall; soon after emerging the larvæ then leave
the gall — in which way I am not able to state; there is no regular previously formed
opening on the capsule, fit for escaping, and no structure in the larva seems specially
adapted for boring through any tissues. Once escaped from the gall the larvæ
probably either settle again in the same host, in which they were raised, or they make
their way out, seeking to infest other individuals of Asteronyx. In the first case they
add to the stock of parasites already present; thus we easily might explain, why some
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