Fig. 5 (from H. Woodward).—1. Limulus polyphemus (dorsal aspect). 2. Limulus, young, in trilobite stage. 3. Prestwichia rotundata. 4. Prestwichia Birtwelli. 5. Hemiaspis limuloides. 6. Pseudoniscus aculeatus.
The appearance of these sea-scorpions is given in Figs. 7 and 8, representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They must have been in those days the tyrants of the deep, for specimens of Pterygotus have been found over six feet in length.
At this time, then, by every criterion hitherto used, by the multitude of species, by the size of individual species, which at this period reached the maximum, by their subsequent decay and final extinction, we must conclude that these forms were in their zenith, that the predominant race at this time was to be found in this group of arthropods. Just previously, the sea swarmed with trilobites, and right into the period when the Gigantostraca flourished, the trilobites are still found of countless forms, of great difference in size. The whole period may be spoken of as the great trilobite age, just as the Tertiary times form the mammalian age, the Mesozoic times the reptilian age, etc. From the trilobites the Gigantostraca and Xiphosura arose, as evidenced by the embryology of Limulus, and, therefore, in the term trilobite age would be included the whole of those peculiar forms which are classified by the names Trilobita, Gigantostraca, Xiphosura, etc. Of all these the only member alive at the present time is Limulus, or the King-Crab.
| Fig. 6.—A Trilobite (Dalmanites) (after Pictet). Dorsal view. | 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 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.
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 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.