170 THE ORIGIN OF VERTEBRATES 



each of the branchiae 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 branchial 

 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 branchiae, 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 winch 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. 



