This has deterred me
from further filling in the outline which I had already sketched.
I will only give, as an example, the probable history of the production
of a single group of Crustacea, and indeed of the most abnormal of all,
the RHIZOCEPHALA, which in the sexually mature state differ so
enormously even from their nearest allies, the Cirripedia, and from
their peculiar mode of nourishment stand quite alone in the entire
animal kingdom.
I must preface this with a few words upon the homology of the roots of
the Rhizocephala, i.e. the tubules which penetrate from its point of
adhesion into the body of the host, ramify amongst the viscera of the
latter, and terminate in caecal branchlets. In the pupae of the
Rhizocephala (Figure 58) the foremost limbs ("prehensile antennae")
bear, on each of the two terminal joints, a tongue-like, thin-skinned
appendage, in which we may generally observe a few small strongly
refractive granules, like those seen in the roots of the adult animal. I
have therefore supposed these appendages to be the rudiments of the
future roots. A perfectly similar appendage, "a most delicate tube or
ribbon," was found by Darwin in free-swimming pupae of Lepas australis
on the last joints of the "prehensile antennae." From the perfect
accordance in their entire structure shown by the pupae of the
Rhizocephala and Cirripedia, there can be no doubt that the appendages
of Sacculina and Lepas, which are so like each other and spring from the
same spot, are homologous structures.
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