3IO AN INTRODUCTION TO THE STUDY OF FOSSILS 



acters : six pairs of limbs about the mouth, the anterior pair 

 chelate ; first pair of abdominal segments forming an operculum, 

 with the genital aperture beneath it ; the following five segments 

 (except the first in Scorpio) respiratory in function ; a ventral 

 shield ; paired median and lateral eyes ; a vibratile spermatozoon 

 (no crustacean except the Cirripedia have such). They agree 

 also in the absence of antennae. The book-gills of the former are 

 replaced by the book-lungs of the latter (probably developed 

 through invagination, this in turn giving rise to the trachea of 

 the other Arachnida). Both have a post-anal spine and in both 

 the anus opens on the segment anterior to this, but this is in 

 Scorpio the twelfth abdominal segment and in Limulus the 

 seventh, the other five being absent. The embryo of the latter, 

 however, w^hen hatched, i.e. during its three-lobed stage, shows 

 the eighth pair well developed and an indication of the ninth. 

 In Neolimulus of the Silurian period there are at least nine 

 abdominal segments, while the Eurypterida (Cambrian to 

 Permian) possess twelve abdominal segments besides the post- 

 anal spine, in this being the exact parallel of Scorpio. 



The eurypterids, Limulus and the scorpions, are thus inti- 

 mately related. The limulids and eurypterids had already 

 diverged in pre-Cambrian times. The scorpions likewise devel- 

 oped very early. The Silurian scorpions, — Palaeophonus, etc., 

 represent apparently the persistence of a group intermediate 

 between the typical modern scorpion and Limulus or Euryp- 

 terus ; this is seen particularly in the appendages. The appar- 

 ent absence of spiracles in these early scorpions indicates that 

 they had not yet taken to a life out of water. This deriva- 

 tion from a life in the water is corroborated in the ontogeny of 

 the living Scorpio, where the book-lungs of the adult are derived 

 from gills borne upon abdominal appendages in the young. Of 

 these three orders the Eurypterida is the most primitive ; but 

 the evidence now at hand apparently indicates that the three 

 have descended from a common ancestor and not from one 

 another. 



