CHAPTER 9. DEVELOPMENTAL HISTORY OF ENTOMOSTRACA, CIRRIPEDES, AND
RHIZOCEPHALA.

The section of the Branchiopoda includes two groups differing even in
their development,--the Phyllopoda and the Cladocera. The latter minute
animals, provided with six pairs of foliaceous feet, which chiefly
belong to the fresh waters, and are diffused under similar forms over
the whole world, quit the egg with their full number of limbs. The
Phyllopoda, on the contrary, in which the number of feet varies between
10 and 60 pairs, and some of which certainly live in the saturated lie
of salterns and natron-lakes, but of which only one rather divergent
genus (Nebalia) is found in the sea,* have to undergo a metamorphosis.
(* If the Phyllopoda may be regarded as the nearest allies of the
Trilobites, they would furnish, with Lepidosteus and Polypterus,
Lepidosiren and Protopterus, a further example of the preservation in
fresh waters of forms long since extinguished in the sea. The occurrence
of the Artemiae in supersaline water would at the same time show that
they do not escape destruction by means of the fresh water, but in
consequence of the less amount of competition in it.) Mecznikow has
recently observed the development of Nebalia, and concludes from his
observations "that Nebalia, during its embryonal life, passes through
the Nauplius- and Zoea-stages, which in the Decapoda occur partly (in
Peneus) in the free state." "Therefore," says he, "I regard Nebalia as a
Phyllopodiform Decapod." The youngest larvae [of the Phyllopoda] are
Nauplii, which we have already met with exceptionally in some Prawns,
and which we shall now find reproduced almost without exception. The
body-segments and feet, which are sometimes so numerous, are formed
gradually from before backwards, without the indication of any
sharply-discriminated regions of the body either by the time of their
appearance or by their form. All the feet are essentially constructed in