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Updated: June 4, 2025
Darwin regards them as forming a peculiar sub-class equivalent to the Podophthalma, Edriophthalma, etc. This appears to me to be most convenient. I would not combine the Rhizocephala with the Cirripedia, as Liljeborg has done, but place them in opposition as equivalent, like the Amphipoda and Isopoda.
It does not seem to me to be probable that two nearly allied species of these social Amphipoda should occur mixed together under the same conditions of life.
It may, however, be remarked, that the different curvature of the embryo in the Amphipoda and Isopoda is so far instructive, as it proves that their present mode of development was adopted only after the separation of these orders, and that, in the primitive stock of the Edriophthalma, the embryos were, if not Nauplii, at least short enough in the body to find room in the egg in an extended position, like the larvae of Achtheres enclosed by the Nauplius-skin.
Cryptogamic vegetation would have the advantage over Phanerogamic; Hydrozoa over Corals; Crustacea over Insecta, and Amphipoda and Isopoda over the higher Crustacea; Cetaceans and Seals over the Primates; the civilization of the Esquimaux over that of the European.
Where, on the contrary, respiration remained with the anterior part of the body, whether in the primitive fashion of Zoea, as in the Tanaides, or by the development of branchiae on the thorax, as in the Amphipoda, the primitive form of the heart was inherited unchanged, because any variations which might make their appearance were rather injurious than advantageous, and disappeared again immediately.
Different as may be the ideas connected with the word "type," no one will dispute that the typical form of the penultimate pair of feet in the Amphipoda is that of a simple ambulatory foot, and not that of a chela, for the latter occurs in no single adult Amphipod; we know it only in the young of the genus Brachyscelus, which therefore in this respect undoubtedly depart more widely than the adults from the type of their order.
Cryptogamic vegetation would have the advantage over Phanerogamic; Hydrozoa over Corals; Crustacea over Insecta, and Amphipoda and Isopoda over the higher Crustacea; Cetaceans and Seals over the Primates; the civilization of the Esquimaux over that of the European.
The similar number of segments* occurring in the Crabs and Macrura, Amphipoda and Isopoda, in which the last seven segments are always different from the preceding ones in the appendages with which they are furnished, could only be regarded as an inheritance from the same ancestors.
Scarcely less striking than the example of the air-breathing Crabs, is the behaviour of the heart in the great section Edriophthalma, which may advantageously be divided, after the example of Dana and Spence Bate, only into two orders, the Amphipoda and the Isopoda.
But so long as it is not demonstrated that our species are particularly in want of this contrivance, or that the latter would rather be injurious than beneficial to other species, its presence only in these few Amphipoda will have to be regarded not as the work of far-seeing wisdom, but as that of a favourable chance made use of by Natural Selection.
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