124 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 from 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 

 branchiae, 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 vertebrae afforded the type of 

 the segmented skeleton, and the anatomists of those days strove hard 

 to resolve the cranio-facial skeleton into a series of modified vertebrae. 

 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 vertebrae 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 



