Which of the different modes of development at present occurring in a class of animals may claim to be that approaching most nearly to the original one, is easy to judge from the above statements. _The primitive history of a species will be preserved in its developmental history the more perfectly, the longer the series of young states through which it passes by uniform steps; and the more truly, the less the mode of life of the young departs from that of the adults, and the less the peculiarities of the individual young states can be conceived as transferred back from later ones in previous periods of life, or as independently acquired._ Let us apply this to the Crustacea. [1] Fig. 65 is drawn from memory, as the little animals, which I at first took for young _Protulae,_ only attracted my attention when I remarked the appearance of the operculum, which induced me to draw them. [2] I will here briefly give my reasons for the opinion that the so-called “complete metamorphosis” of Insects, in which these animals quit the egg as grubs or caterpillars, and afterwards become quiescent pupæ incapable of feeding, was not inherited from the primitive ancestor of all Insects, but acquired at a later period. The order Orthoptera, including the Pseudoneuroptera (Ephemera, _Libellula,_ etc.) appears to approach nearest to the primitive form of Insects. In favour of this view we have:— 1. The structure of their buccal organs, especially the formation of the labium, “which retains, either perfectly or approximately, the original form of a second pair of maxillæ” (Gerstäcker). 2. The segmentation of the abdomen; “like the labium, the abdomen also very generally retains its original segmentation, which is shown in the development of eleven segments” (Gerstäcker). The Orthoptera with eleven segments in the abdomen, agree perfectly in the number of their body-segments with the Prawn-larva represented in Fig. 33, or indeed, with the higher Crustacea (Podophthalma and Edriophthalma) in general, in which the historically youngest last thoracic segment (see Chapter 12), which is sometimes late-developed, or destitute of appendages, or even deficient, is still wanting. 3. That, as in the Crustacea, the sexual orifice and anus are placed upon different segments; “whilst the former is situated in the ninth segment, the latter occurs in the eleventh” (Gerstäcker). 4. Their palæontological occurrence; “in a fossil state the Orthoptera make their appearance the earliest of all Insects, namely as early as the Carboniferous formation, in which they exceed all others in number” (Gerstäcker). 5. The absence of uniformity of habit at the present day in an order so small when compared with the Coleoptera, Hymenoptera, etc. For this also is usually a phenomenon characteristic of very ancient groups of forms which have already overstepped the climax