THE EVIDENCE OF THE RESPIRATORY APPARATUS 15 I 



breathed is unknown ; it shows no signs of stigmata, such as exist in 

 the scorpion of to-day. 



Although we possess as yet no certain knowledge of the position 

 of the gill-openings in these ancient scorpion-like forms, what we 

 can say with certainty — and that is the important fact — is, that at 

 the time when the vertebrates appeared, a very large number of the 

 dominant arthropod race possessed internally-situated branchife, which 

 had been directly derived from the branchiae-bearing appendages of 

 their Limulus-like kinsfolk. 



This abolition of the branchiie-bearing appendages as external 

 organs of locomotion, with the retention of the important branchial 

 portion of the appendage as internal branchiae, is a very important 

 suggestion in any discussion of the way vertebrates have arisen from 

 arthropods; for, if the same principle is of universal application, it 

 leads directly to the conclusion that whenever an appendage possesses 

 an organ of vital importance to the animal, that organ will remain, 

 even though the appendage as such completely vanishes. Thus, as 

 will be shown later, special sense-organs such as the olfactory remain, 

 though the animal no longer possesses antennae ; the important ex- 

 cretory organs, the coxal glands, and important respiratory organs, 

 the branchiae, are still present in the vertebrate, although the appen- 

 dages to which they originally belonged have dwindled away, or, at 

 all events, are no longer recognizable as arthropod appendages. 



Innervation of Beanchial Segments. 



Passing from a priori considerations to actual facts, it is advisable 

 to commence with the innervation of the branchial segments ; for, 

 seeing that the foundation of the whole of this comparative study 

 of the vertebrate and the arthropod is based upon the similarity of 

 the two central nervous systems, it follows that we must look in 

 the first instance to the innervation of any organ or group of organs 

 in order to find out their relationship in the two groups of animals. 



The great characteristic of the vertebrate branchial organs is their 

 segmental arrangement and their innervation by the vagus group of 

 nerves, i.e. by the hindermost group of the cranial segmental nerves. 

 These cranial nerves are divided by Gegenbaur into two great groups 

 — an anterior group, the trigeminal, which supplies the muscles of 

 mastication, and a posterior group, the vagus, which is essentially 



