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, 



Fig. 6.— A Trilobite (Dalma- 

 tites) (after Pictet). Dorsal 



Fig. 7. — Euryplerus remises (after 

 Nieskowski). 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 



