i5o 



THE ORIGIN OF VERTEBRATES 



occurred in that lower group. On the assumption, therefore, that 

 the vertebrate branchiae represent the branchial portion of the 

 arthropod mesosomatic appendages which have sunk in and so 

 become internal, we ought to find that in members of this very 

 group such inclusion of branchial appendages has taken place. This, 

 indeed, is exactly what we do find, for in all the scorpion tribe, which 

 is acknowledged to be closely related to Limulus, there are no 



external mesosomatic appendages, 

 but in all cases these appendages 

 have sunk into the body, have 

 disappeared as such, and retained 

 only the vital part of them — the 

 branchiae. In this way the so-called 

 lung - books of the scorpion are 

 formed, which are in all respects 

 homologous with the branchiae or 

 gill-books of Limulus. Now, as 

 already mentioned, the lords of 

 creation in the palseostracan times 

 were the sea-scorpions, which, as is 

 seen in Fig. 62, resembled the land- 

 scorpions of the present day in the 

 entire absence of any external ap- 

 pendages on the segments of the 

 mesosomatic region. As they lived 

 in the sea, they must have breathed 

 with gills, and those branchial ap- 

 pendages must have been internal, 

 just as in the land-scorpions of the 

 present time. Indeed, markings 

 have been found on the internal 

 side of the segments 1-5, Fig. 62, which are supposed to indicate 

 branchiffi, and these segments are therefore supposed to have borne 

 the branchire. Up to the present time no indication of gill-slits 

 has been found, and we cannot say with certainty how these 

 animals breathed. Further, in the Upper Silurian of Lesmahago, 

 Lanarkshire, a scorpion (Palccoijhonus Hunteri), closely resembling the 

 modern scorpion, has been found, which, as Lankester states, was in 

 all probability aquatic, and not terrestrial in its habits. How it 



Fig. 62. — Eurypterus. 



The segments and appendages on the 

 right are numbered in correspon- 

 dence with the cranial system of 

 lateral nerve-roots as found in verte- 

 brates. lf.,metastoma. The surface 

 ornamentation is represented on 

 the first segment posterior to the 

 branchial segments. The opercular 

 appendage is marked out by dots. 



