VERTEBRATES. 



21 



Fig. 25. — Lower jaw and liyoid arch of salmon ; an, angular ; ar, 

 articular ; bb, basibranclilostegal ; &r, branchiostegai rays ; c, cera- 

 tohyal ; d, dentary ; e, epicersLtohyal ; g^ glossohyal ; A, liypohyal ; 

 i, internyal ; q, quudrate ; s, symplectic. 



ends of the visceral arches ; the first of these is the basihyal, while the succeeding are 

 styled basibi-anchials. In most forms the basihyal attains greater importance from its 

 supporting the tongue, into which it sends forwards an entoglossal or glossohyal pro- 

 cess. It is to this relationship to 

 the tongue, that the hyoid arch 

 owes its name, and its persistence 

 throughout the higher forms. 

 The lower part of the arch 

 retains its connection, with the 

 upper part, in fishes, by means 

 of an interhyal piece, between 

 which and the basihyal are gen- 

 erally found epiceratoliyal, cera- 

 tohyal, and hypohyal pieces (Fig. 

 25). In the higher forms, where 

 the upper part of the arch has become subservient to the auditory organ, the lower 

 part is attached to the auditory region of the skull by a tym- 

 panohyal, between which and the basihyal the following ossifi- 

 cations may occur: stylohyal, epihyal, ceratohyal; but some 

 or all of these may remain unossified. When present they 

 are known as the anterior cornua of the hyoid bone (Fig. 26). 

 The posterior cornua of the hyoid are the remains of the first 

 branchial arches, and may enter into important relations with the 

 larynx. 



The basibranchials are rarely independently represented, ex- 

 cept in fishes, where, however, they are frequently coalesced. 

 In the arches themselves, the following separate pieces are gen- 

 erally present (Fig. 24) : hypobranchials, ceratobranchial, epi- 

 branchials and pharyngobranchials, the latter being loosely at- rre. 26.— Hyoid appara- 

 tached to the ventral face of the skull, and generally canwiug ceratobranchial; c, first 



, , ^ , ., , . , , basibranohial : e, cera- 



superior pharyngeal cement bones on them, which are covered tohyai ; h, second basi- 

 with teeth. In most bony fishes the fifth arch supports no gill, oWaj! '^ ' ^' ^'" ™"" 

 but bears on the surface, turned towards the mouth, considerable dentigerous cement 

 plates — the inferior pharyngeal bones. 



The Skeleton of the Limes. 



Two systems of locomotor appendages are distinguished in the vertebrates — a 

 median unpaired system confined to the fishes and aquatic amphibians, and the two 

 paired anterior and posterior appendages, typical of the whole branch. In certain 

 primitive forms the median unpaired fin may be an almost continuous fold occupying the 

 dorsal and ventral middle line, but the tendency is towards its restriction to certain tracts 

 which will be treated of in detail in the special description of the fishes. In this 

 class alone is the fold supported by fin-rays; these in their turn rest on bones, 

 which are either wedged in between the spines of the vertebral column (hence inter- 

 spinous bones), or are continuous with these and thus answer to the segmentation of 

 the body. They may be present although the corresponding tract of unpaired fin has 

 disappeared. 



According to the most prevalent theory the paired fins have become developed by 



