26 THE ORIGIN OF VERTEBRATES 
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, 
Fic. 6.—A Trinosire (Dalma- Fic. 7.--Euryplerus remipes (after 
tites) (after Prctet). Dorsal Niesxowsk1), Dorsal view. 
view. 
Gigantostraca, Xiphosura, etc. Of all these the only member alive 
at the present time is Limulus, or the King-Crab. 
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 - 
