As not only the _Tanaides_ (which reasons elsewhere stated (_vide suprà_) justify us in regarding as particularly nearly related to the primitive Isopod) and the Amphipoda, but also the Decapod Crustacea, possess a heart with three pairs of fissures essentially in the same position; and as the same position of the heart recurs (_vide infrà_) even in the embryos of the Mantis-Shrimps (_Squilla_), in which the heart of the adult animal, and even, as I have elsewhere shown, that of the larvæ when still far from maturity, extends in the form of a long tube with numerous openings far into the abdomen, we must unhesitatingly regard the heart of the Amphipoda as the primitive form of that organ in the Edriophthalma. As, moreover, in these animals the blood flows from the respiratory organs to the heart without vessels, it is very easy to see how advantageous it must be to them to have these organs as much approximated as possible. We have reason to regard as the primitive mode of respiration, that occurring in _Tanais_ (_vide suprà_). Now, where, as in the majority of the Isopoda, branchiæ were developed upon the abdomen, the position and structure of the heart underwent a change, as it approached them more nearly, but without the reproduction of a common plan for these earlier modes of structure, either because this transformation of the heart took place only after the division of the primary form into subordinate groups, or because, at least at the time of this division, the varying heart had not yet become fixed in any new form. Where, on the contrary, respiration remained with the anterior part of the body,—whether in the primitive fashion of Zoëa, as in the _Tanaides,_ or by the development of branchiæ 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.