From what lower group the _Campodea_ type was itself derived is a question of great difficulty. Fritz Müller indeed says,[64] “if all the classes of Arthropoda (Crustacea, Insecta, Myriopoda, and Arachnida) are indeed all branches of a common stem (and of this there can scarcely be a doubt), it is evident that the water-inhabiting and water-breathing Crustacea must be regarded as the original stem from which the other terrestrial classes, with their tracheal respiration, have branched off.” Haeckel, moreover, is of the opinion that the Tracheata are developed from the Crustacea, and probably from the Zoëpoda. For my own part, though I feel very great diffidence in expressing an opinion at variance with that of such high authorities, I am rather disposed to suggest that the _Campodea_ type may possibly have been derived from a less highly developed one, resembling the modern Tardigrade,[65] a (Fig. 56) smaller and much less highly organized being than _Campodea_. It possesses two eyes, three anterior pairs of legs, and one at the posterior end of the body, giving it a curious resemblance to some Lepidopterous larvæ. [Illustration: FIG. 56, Tardigrade (after Dujardin).] These legs, however, as will be seen, are reduced to mere projections. But for them, the Tardigrada would closely resemble the vermiform larva so common among insects. Among Trichoptera the larva early acquires three pairs of legs, but as Zaddach has shown,[66] there is a stage, though it is quickly passed through, in which the divisions of the body are indicated, but no trace of legs is yet present. Indeed, there appear to be reasons for considering that while among Crustacea the appendages appear before the segments, in Insects the segments precede the appendages, although this stage of development is very transitory, and apparently, in some cases, altogether suppressed. I say “apparently,” because, as I have already mentioned, I am not yet satisfied that it will not eventually be found to be so in all cases. Zaddach, in his careful observations of the embryology of _Phryganea_, only once found a specimen in this stage, which also, according to the researches of Huxley,[67] seems to be little more than indicated in _Aphis_. It is therefore possible that in other cases, when no such stage has been observed, it not really may be absent, but, from its transitoriness, may have hitherto escaped attention. Fritz Müller has expressed the opinion[68] that this vermiform type is of comparatively recent origin. He says: “The ancient insects approached more nearly to the existing Orthoptera, and perhaps to the wingless Blattidæ, than to any other order, and the complete metamorphosis of the Beetles, Lepidoptera, &c., is of later origin.” “There were,” he adds, “perfect insects before larvæ and pupæ.” This opinion has been adopted by Mr. Packard[69] in his “Embryological Studies on Hexapodous Insects.”