THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 51 
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 branchia, which 
had been directly derived from the branchie-bearing appendages of 
their Limulus-like kinsfolk. 
This abolition of the branchie-bearing appendages as external 
organs of locomotion, with the retention of the important branchial 
portion of the appendage as internal branchie, 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 antenne; the important ex- 
eretory organs, the coxal glands, and important respiratory organs, 
the branchiee, 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 BRANCHIAL 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, 7.¢. 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 
