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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.
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 civilisation of the Esquimaux over that of the European.
Considering this uniformity presented by the heart in the entire order of the Amphipoda, it cannot but seem very remarkable, that in the very next order of the Isopoda, we find it to be one of the most changeable organs. Heart of a young Cassidina. Heart of a young Anilocra. Abdomen of the male of Entoniscus Cancrorum. h. Heart. l. In all other Isopoda the heart is removed towards the abdomen.
Considering the common opinion as to the distribution of the Amphipoda, namely, that they increase in multiplicity towards the poles, and diminish towards the equator, it may seem strange that I speak of a considerable number of species on a subtropical coast. In systematising from single dead specimens, as to the sex, age, etc. of which nothing is known, similar errors are unavoidable.
On the other hand the uniformity of development that prevails in each of the two orders which is expressed in the Amphipoda for example in the formation of the "micropylar apparatus," in the Isopoda in the want of the last pair of ambulatory feet testifies that the present mode of development has come down from a very early period and extends back beyond the separation of the present families.
It is evident that the larva stands essentially in the grade of Zoea. Less varied than that of the Stalk-eyed Crustacea is the mode of development of the Isopoda and Amphipoda, which Leach united in the section Edriophthalma, or Crustacea with sessile eyes. Embryo of Ligia in the egg, magnified 15 diam.
Under the latter supposition its isolated occurrence is intelligible, whilst we cannot perceive why the Creator blessed just these few species with an apparatus which he found to be quite compatible with the "general plan of structure" of the Amphipoda, and yet denied it to others which live under the same external conditions, and equal them even in their extraordinary salacity.
Does it not necessitate that we should unite as the descendants of the same primitive ancestors, Mysis with the Isopoda on the one hand, and on the other the rest of the Podophthalma with the Amphipoda? I think not. Such a necessity exists only for those who estimate a peculiarity at a higher value because it makes its appearance at an earlier period of the egg-life.
Among the males of these various Bopyridae, that of Entoniscus Porcellanae occupies the lowest place; it is confined all its life to six pairs of feet, which are reduced to shapeless rounded lumps. The Amphipoda are distinguishable from the Isopoda at an early period in the egg by the different position of the embryo, the hinder extremity of which is bent downwards.
And if at the present day the majority of the Crabs and Macrura, and indeed the Stalk-eyed Crustacea in general, pass through Zoea-like developmental states, and the same mode of transformation was to be ascribed to their ancestors, the same thing must also apply, if not to the immediate ancestors of the Amphipoda and Isopoda, at least to the common progenitors of these and the Stalk-eyed Crustacea.
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