thrown off the previously existing natatory feet, whilst in the other
families these appendages persist, more or less altered. "Beyond this
stage of free development, many forms of the parasitic Copepoda, such as
Lernanthropus and Chondracanthus, do not pass, as they do not acquire
the third and fourth pairs of limbs, nor does a separation of the fifth
thoracic segment from the abdomen take place; others (Achtheres) even
fall to a lower grade by the subsequent loss of the two pairs of
natatory feet. But all free Copepoda, and most of the parasitic
Crustacea, pass through a longer or shorter series of stages of
development, in which the limbs acquire a higher degree of division into
joints in continuous sequence, the posterior pairs of feet are
developed, and the last thoracic segment and the different abdominal
segments are successively separated from the common terminal portion."

(FIGURE 55. Nauplius of Tetraclita porosa after the first moult,
magnified 90 diam. The brain is seen surrounding the eye, and from it
the olfactory filaments issue; behind it are some delicate muscles
passing to the buccal hood.)

There is only one thing more to be indicated in the developmental
history of the parasitic Crustacea, namely that some of them, such as
Achtheres percarum, certainly quit the egg like the rest in a
Nauplius-like form, inasmuch as the plump, oval, astomatous body bears
two pairs of simple rowing feet, and behind these, as traces of the
third pair, two inflations furnished each with a long seta, but that
beneath this Nauplius-skin a very different larva lies ready prepared,
which in a few hours bursts its clumsy envelope and then makes its
appearance in a form "which agrees in the segmentation of the body and
in the development of the extremities with the first Cyclops-stage"
(Claus). The entire series of Nauplius-stages which are passed through