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The simple median eye appears earlier, and would therefore be more important than the compound paired eyes; the scale of the antennae in the Prawns would be more important than the flagellum; the maxillipedes of the Decapoda would be more important than the chelae and ambulatory feet, and the anterior six pairs of feet in the Isopoda, than the precisely similarly formed seventh pair; in the Amphipoda the most important of all organs would be the "micropylar apparatus," which disappears without leaving a trace soon after hatching; in Cyclops the setae of the tail would be more important than all the natatory feet; in the Cirripedia the posterior antennae, as to which we do not know what becomes of them, would be more important than the cirri, and so forth.
Now as to the question: How were Cirripedia converted by natural selection into Rhizocephala? A considerable number of existing Cirripedia settle exclusively or chiefly upon living animals; on Sponges, Corals, Mollusks, Cetaceans, Turtles, Sea-Snakes, Sharks, Crustaceans, Sea Urchins, and even on Acalephs.
The ANNULOSA. The Carboniferous Insecta and Arachnida are neither less specialized, nor more embryonic, than these that now live, nor are the Liassic Cirripedia and Macrura; while several of the Brachyura, which appear in the Chalk, belong to existing genera; and none exhibit either an intermediate, or an embryonic, character.
Had Darwin died before "The Origin of Species" was published, he would have been famous among scientific men, although it was the abuse of theologians on the publication of "The Origin of Species" that really made him world-famous. Alfred Russel Wallace, Darwin's chief competitor said that "A Monograph on the Cirripedia" is enough upon which to found a deathless reputation.
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.
As the Cirripedia and Rhizocephala now in general resemble each other far more than in their Nauplius-state, this is also the case with the individual members of each of the two orders.
The sequence of the sections of the body in order of time seems originally to have been, that first the fore-body, then the hind-body, and finally the middle-body was formed. This is a circumstance which renders very doubtful the equivalence of the middle-body of the Malacostraca with the section of the body which in the Copepoda bears the swimming feet and in the Cirripedia the cirri.
It seems probable that the swimming feet of the Copepoda, as also of the pupae of Cirripedia and Rhizocephala, represent the abdominal feet of the Malacostraca, that is to say, are derived by inheritance from the same source with them.
Yet before these were published he had issued "A Journal of Research into Geology and Natural History," "The Zoology of the Voyage of the 'Beagle," "A Treatise on Coral Reefs, Volcanic Islands, Geological Observations," and "A Monograph of the Cirripedia."
From this it follows that this appendage in Lepas australis can hardly be anything but a young cement-duct. If, therefore, the supposition that the appendages on the antennae of the pupae of Rhizocephala are young roots be correct, the roots of the Rhizocephala are homologous with the cement-ducts of the Cirripedia.
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