150 THE ORIGIN OF VERTEBRATES 
occurred in that lower group. On the assumption, therefore, that 
the vertebrate branchie 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 
branchie. In this way the so-called 
lung- books of the scorpion are 
formed, which are in all respects 
homologous with the branchiz or 
gill-books of Limulus. Now, as 
already mentioned, the lords of 
creation in the palzostracan times 
were the sea-scorpions, which, as is 
seen in Fig. 62, resembled the Jand- 
scorpions of the present day in the 
entire absence of any external ap- 
The segments and appendages on the pendages “ the seater of the 
-right are numbered in correspon. mesosomatic region. As they lived 
dence with the cranial system of in the sea, they must have breathed 
ierneeorca fog inet with gills and those branchial ap- 
ornamentation is represented on pendages must have been internal, 
the first segment posterior to the just as in the land-scorpions of the 
branchial segments. The opercular . ieee 
appendage is marked out by dots. present time. Indeed, markings 
have been found on the internal 
side of the segments 1-5, Fig. 62, which are supposed to indicate 
branchize, and these segments are therefore supposed to have borne 
the branchie. 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 (Palwophonus Huntert), 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 
Fic. 62.—Eurypterus. 
