772 AETHROPODA phylum vii 



* 



having such appendages. Also it is to be inferred that the cephalothorax 

 in the embryo of the Scorpion retains ancestral features, from the facts that 

 its length corresponds to about six abdominal segments and it equals the 

 latter in width. 



A comparison of the larvae of all three, Eurypterids, Limulus and the 

 Scorpion, shows that the two last-named have lost the primitive form of the 

 abdomen by acceleration ; that of Limulus being much broadened, that of 

 the Scorpion abruptly contracted to the tail or postabdomen, while the 

 Eurypterids have best preserved the original gradual and uniform contraction. 

 The carapaces, of Eurypterids and the Scorpion have most nearly retained 

 the original proportions and form of the common ancestor. Of the cephalo- 

 thoracic appendages the chelicerae are alike in all three groups and obviously 

 ancestral in their form ; the remaining legs have taken quite different courses 

 of adaptation, the Scorpions having developed the powerful chelate pedipalps, 

 the Eurypterids the swimming legs, while those of Limulus have remained 

 relatively undifferentiated. 



These and other facts tend to support the inference that neither Limulus 

 nor the Scorpions are derivable from Eurypterids, but that all three, while 

 related, have early separated ; and that the Eurypterids are still nearest in 

 their general aspect to the early common ancestor. The appearance of 

 Eurypterids in the Cambrian with the essential characters of the group is 

 in accordance with their larval aspect, while the early separation of Scorpions 

 from the primitive stock is evinced by the occurrence of typical Scorpions in 

 the Silurian, and by the fact that in the Carboniferous they show a greater 

 diversity of form than they do to-day. On the other hand the similarity of 

 the ancient Palaeophonus nuntius to Recent forms is conclusive evidence that 

 the Scorpions have been very " persistent types " and had developed their 

 typical characters much earlier than the Silurian. There is no reason to 

 doubt that, as there are Eurypterids in the Cambrian, the Scorpions also 

 reach back to that era, and the diversion from the common ancestor must 

 have already been inaugurated in early Cambrian time. 



As to what this common ancestor was we have no clue. Surely the 

 Trilobites, which are true primitive Crustacea, are not ancestrally or other- 

 wise closely related to Merostomes, and the latter even in the Cambrian are 

 far removed from any possible synthetic ancestors, as is shown by their very 

 definite number of segments and the arrangement of their appendages. We 

 must search, therefore, for still more primitive Arthropods than the Crustacea 

 as ancestors of Merostomes and ilrachnids generally. In support of this view 

 Clarke and Ruedemann point to the absence of anything in the ontogeny of 

 the Eurypterids that would suggest a crustacean nauplius stage, the admitted 

 absence of all crustacean characters in the adult forms, and the equal absence 

 of all crustacean features in the ontogenies of Limulus and the Scorpion. 



Order 1. XIPHOSURA Gronovius.i 



Body, in mature types, distinctly trilohed longitudinally. Cephalothorax large, 

 semicircular, the compound eyes, when present, laterally situated, and ocelli near the 



^ Literature : Hoeven, J. van dei\ Recherclies sur lliistoire iiaturelle et ranatoniie des Limules. 

 Leydeii, 1838. — Minister, G. Oraf zu, Beitriige zur Petrefaktenkunde. Parts i., iii., 1840. — 

 Gegenhaur, C, Anatomische Untersuchuugen eiiies Limixlus. Abliaudl. iiaturf. Ges. Halle, 18.58. 



