E24. THE ORIGIN OF VERTEBRATES 
Their remnants become more or less important parts of the hyoid 
bone, and are employed solely in support of the tongue. 
In no single animal is there any evidence that the foremost arch, 
the mandibular, is a true branchial arch. As low down as the 
Elasmobranchs it becomes divided into two elements which form 
respectively the upper and lower jaws; the hyoid arch, on the other 
hand, although it has altered its form and acquired the secondary 
function of supporting the mandibular arch, still retains its respi- 
ratory function. 
The evidence afforded by the mode of formation of the skeletal 
tissues of vertebrates down to the Elasmobranchs indicates that the 
primitive cranial skeleton arose froni two paired basal cartilages, the 
parachordals and trabecule, to which were attached respectively 
cartilaginous cases enclosing the organs of hearing and smell. In 
addition, the branchial portion of the cranial region was provided 
with cartilaginous bars arranged serially for the support of the 
branchie, with the exception of the foremost, the mandibular bar, 
which formed supporting tissues for the mouth—the upper and 
lower jaws. 
Just as in past times the spinal nerves and the segments they 
supplied were supposed to represent the type on which the original 
vertebrate was built, so also the spinal vertebre afforded the type of 
the segmented skeleton, and the anatomists of those days strove hard 
to resolve the cranio-facial skeleton intoa series of modified vertebre. 
Owing especially to the labours of Huxley, who showed that the seg- 
mentation in the head-region was essentially a segmentation due to 
the presence of branchial bars, this conception was finally laid to rest 
and nowadays it is admitted to be hopeless to resolve the cranium 
into vertebral segments. Still, however, the vertebrate is a segmented 
animal and its segmented nature is visible in the cranial region, so far 
as the skeletal tissues are concerned, in the shape of the series of 
branchial and visceral bars. 
To this segmentation the name of ‘ branchiomeric’ has been given, 
while that due to the presence of vertebre is called ‘ mesomeric.’ 
As we have seen, the internal bony skeleton of the vertebrate 
commences as.a cartilaginous and membranous skeleton. For this 
reason the preservation of such skeletons is impossible in the fossil 
form, unless the cartilage has become impregnated with lime salts, 
so that there is but little hope of ever obtaining traces of such 
