According to all the characters established in the last paragraph, the Prawn that we traced from the Nauplius through states analogous to Zoëa and _Mysis_ to the form of a Macrurous Crustacean appears at present to be the animal, which in the section of the higher Crustacea (Malacostraca) furnishes the truest and most complete indications of its primitive history. That it is the most complete is at once evident. That it is the truest must be assumed, in the first place, because the mode of life of the various ages is less different than in the majority of the other Podophthalma; for from the Nauplius to the young Prawn they were found swimming freely in the sea, whilst Crabs, _Porcellanæ,_ the Tatuira, _Squilla,_ and many Macrura, when adult usually reside under stones, in the clefts of rocks, holes in the earth, subterranean galleries, sand, etc., not to mention other deviations in habits such as are presented by the Hermit Crabs, _Pinnotheres,_ etc.,—and secondly and especially because the peculiarities which distinguish the Zoëa of this species particularly from other Zoëæ (the employment of the anterior limbs for swimming, the furcate tail, the simple heart, the deficiency of the paired eyes and abdomen at first, etc.) are neither to be deduced from a retro-transfer of late-acquired advantages to this early period of life, nor to be regarded at all as advantages over other Zoëæ which the larva might have acquired in the struggle for existence. A similar development must have been once passed through by the primitive ancestor of all Malacostraca, probably differing from that of our Prawn, especially in the circumstance that it would go on more uniformly without the sudden change of form and mode of locomotion produced in the latter by the simultaneous sprouting forth and entering into action in the Nauplius of four and in the Zoëa of five pairs of limbs. It is to be supposed that, not only originally but even still, in the larvæ of the first Malacostraca, the new body-segments and pairs of limbs are formed singly,—first of all the segments of the fore-body, then those of the abdomen, and finally those of the middle-body,—and, moreover, that in each region of the body the anterior segments were formed earlier than the posterior ones, and therefore last of all the hindermost segment of the middle-body. Of this original mode more or less distinct traces still remain, even in species in which, in other respects, the course of development of their ancestors is already nearly effaced. Thus the abdominal feet of the Prawn-larva represented in Fig. 33, are formed singly from before backwards, and after these the last feet of the middle-body; thus, in _Palinurus,_ the last two pairs of feet of the middle-body are formed later than the rest; thus in the young larvæ of the Stomapoda the last three abdominal segments are destitute of limbs, which are still wanting on the last of them in