[Illustration: FIG. 6.--A TRILOBITE (_Dalmanites_) (after PICTET). Dorsal view.]
[Illustration: FIG. 7.--_Eurypterus remipes_ (after NIESKOWSKI). Dorsal view.]
As, however, the term 'trilobite' does not include the members of the king-crab or sea-scorpion groups, it is advisable to use some other term to represent the whole group. They cannot be called crustaceans or arachnids, for in all probability they gave origin to both; the nearest approach to the Trilobite stage of development at the present time is to be found perhaps in Branchipus (Fig. 10) and Apus (Fig. 9), just as the nearest approach to the Eurypterid { 27}form is Limulus. Crustaceans such as crabs and lobsters are of much later origin, and do not occur in any quantity until the late Mesozoic period. The earliest found, a kind of prawn, occurs in the Carboniferous age.
[Illustration: FIG. 8.--A, _Pterygotus Osiliensis_ (from SCHMIDT). B, _Stylonurus Logani_ (from WOODWARD). C, _Slimonia acuminata_ (from WOODWARD).]
Korschelt and Heider have accordingly suggested the name _Palæostraca_ for this whole group, and _Protostraca_ for the still earlier { 28}arthropod-like animals which gave origin to the trilobites themselves. This name I shall adopt, and speak, therefore, of the _Palæostraca_ as the dominant race at the time when vertebrates first appeared.
If, then, there is no break in the law of evolution here, the race which was predominant at the time when the vertebrate first appeared must have been that from which the first fishes arose, and these fishes must have resembled, not the crustacean proper, or the arachnid proper, but a member of the palæostracan group. Moreover, just as the Labyrinthodonts show special affinities to the fishes which were then living, so we should expect that the forms of the earliest fish would resemble the arthropodan type dominant at the time more closely than the fish of a later era.
At first sight it seems too great an absurdity even to imagine the possibility of any genetic connection between a fish and an arthropod, for to the mind's eye there arises immediately the picture of a salmon or a shark and a lobster or a spider. So different in appearance are the two groups of animals, so different their methods of locomotion, that it is apparently only an inmate of a lunatic asylum who could possibly suggest such a connection. Much more likely is it that a fish-like form should have been developed out of a smooth, wriggling, worm-like animal, and it is therefore to the annelids that the upholders of the theory of the reversal of surfaces look for the ancestor of the vertebrate.