From this "egg-like larva"--(Darwin says of
it, "I hardly know what to call it")--the pupa is directly produced. Its
carapace is but slightly compressed laterally and hairy, as in Sacculina
purpurea; the adherent feet are of considerable size, and the natatory
feet are wanting, as, in the adult animal, are the corresponding cirri.
As I learn from Mr. Spence Bate, the Nauplius-stage appears to be
overleaped and the larvae to leave the egg in the pupa-form, in the case
of a Rhizocephalon (Peltogaster ?) found by Dr. Powell in the Mauritius.
(FIGURES 61 TO 63. Eggs of Tetraclita porosa in segmentation, magnified
90 diam. The larger of the two first-formed spheres of segmentation is
always turned towards the pointed end of the egg.
FIGURE 64. Egg of Lernaeodiscus Porcellanae, in segmentation, magnified
90 diam.)
I will conclude this general view with a few words upon the earliest
processes in the development of the Crustacea. Until recently it was
regarded as a general rule that, by the partial segmentation of the
vitellus a germinal disc was formed, and in this, corresponding to the
ventral surface of the embryo, a primitive band. We now know that in the
Copepoda (Claus), in the Rhizocephala (Figure 64), and, as I can add, in
the Cirripedia (Figures 61 to 63) the segmentation is complete, and the
embryos are sketched out in their complete form without any preceding
primitive band.
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