170 THE ORIGIN OF VERTEBRATES 
each of the branchiz of the scorpion group is directly compared 
‘with the branchial part of the Limulus appendage which has sunk 
into and amalgamated with the ventral surface. 
According to this view, the modification which has taken place in 
transforming the branchial Limulus-appendage into the brancbial 
scorpion-appendage is a further stage of the process by which the 
Limulus branchial appendage itself has been formed, viz. the getting 
rid of the free locomotor segments of the original appendage, thus 
confining the appendage more and more to the basal branchial 
portion, So far has this process been carried in the scorpion that 
all the free part of the appendage has disappeared; apparently, also, 
the intrinsic muscles of the appendage have vanished, with the 
possible exception of the post-stigmatic muscle, so that any direct 
comparison between the branchial appendages of Limulus and the 
scorpions is limited to the comparison of their branchie, their nerves, 
and their afferent and efferent blood-vessels. 
In the case of Ammoccetes the comparison must be made not 
with air-breathing but with water-breathing scorpions, such as 
existed in past ages in the forms of Eurypterus, Pterygotus, Slimonia, 
and with the crowd of trilobite and Limulus-like forms which were 
in past ages so predominant in the sea; forms in some of which the 
branchial appendages had already become internal, but which, from 
the very fact of these forms being water-breathers, probably 
resembled, in respect of their respiratory apparatus, Limulus rather 
than the present-day scorpion. 
On the assumption that the branchial appendages of Ammoccetes, 
like the branchial appendages of the scorpion group, are to a certain 
extent comparable with those of Limulus, it becomes a matter of great 
interest to inquire whether the mode in which respiration is effected 
in Ammoccetes resembles most that of Limulus or of the scorpion. 
THE ORIGIN OF THE BRANCHIAL MUSCULATURE. 
The difference between the movements of respiration in Limulus 
and those of the scorpions consists in the fact that, although in both 
cases respiration is effected mainly by dorso-ventral muscles, these 
muscles are not homologous in the two cases: in the former, the 
dorso-ventral appendage-muscles are mainly concerned, in the latter, 
the dorso-ventral somatic muscles. 
