BRANCHIAL SKELETON OF VERTEBRATA. 4G9 



of arches either retain their primitive function of being supports 

 for the branchial arches, or undergo a number of modifications. 



All these arches evidently had the same original function. Their 

 relation to the respiratory apparatus has not disappeared in the first 

 pair only, which has been converted into jaws ; the hinder arches 

 also have gradually lost their functional and anatomical characters ; 

 it is, therefore, reasonable to suppose that this is but the last of a 

 series of reductions, which first commenced on a much larger 

 number of arches. If this is so, the branchial skeleton of the 

 Craniota is the remnant of an apparatus, in which there were 

 primitively a far larger number of arches. This view is supported 

 by a comparison with Amphioxus, as well as by the considerations 

 to which we are led by a study of the branchial apparatus and of 

 the peripheral nervous system. 



As we pass through the Fishes to the Amphibia we may note 

 how this apparatus gradually loses its primitive relations ; while in 

 the Reptilia and all higher forms it has no relation at all to the 

 respiratory organs. 



§ 354. 



All the branchial arches are united ventrally by azygos pieces— 

 the copula?. The various arches are always segmented into a 

 number of pieces, which are generally movably united with one 

 another. The upper portion of the hyoid, as above described, as well 

 as of the mandibular arch, enters into relations with the cranium; 

 these arches thus lose all connection with the other arches, with which 

 the lower portion only of the hyoid arch is still connected. 



The succeeding arches are either slightly connected with, or 

 only indirectly united to, the cranium ; this is effected by their 

 being attached to the base of the skull, or, when their point of 

 attachment is more extended, to the commencement of the vertebral 

 column. In many Selachii the hyoid arch has the same conformation 

 as the branchial ones (Fig. 255, II). As a rule its copula is in- 

 creased in size, and affords a suppoi^t for the tongue. In the 

 Selachii and Chimaeras this arch retains its primitive function of a 

 branchiferous portion of the skeleton. This relation disappears in 

 the Ganoi'dei and Teleostei, where this gill is rudimentary, and the 

 rays of the upper portion, which is converted into the hyomandi- 

 bular and symplectic, are represented by the opercular apparatus 



(§ 345). 



The lower portion of the hyoid arch, or true hyoid, has then 

 bony instead of cartilaginous rays (Fig. 256, I r, branchiostegal 

 rays), and a membrane extends between them which covers over the 

 whole of the branchial apparatus. The hyoid arch thus develops 

 an organ of defence for the respiratory apparatus. 



There are five pairs of arches connected with the respiratory 

 apparatus; occasionally there are six or seven (Notidani). No more 

 than five are ever found in the Osseous Fishes. While the 

 anterior arches (I II III) are always provided with copulas (/ g), 



