ORGANS OF RESPIRATION. 245 



the respiratory accessory gill receives dark-colored blood from the 

 branchial artery. A similar pseudobranchia is possessed by the Rays 

 and Sharks upon the valve of the anterior spiracle. 



ORGANS OF RESPIRATION. 



ALL Fishes respire by means of branchiae or gills, for the support 

 and protection of which organs we find a very complicated frame- 

 work of bone or cartilage developed, and exhibiting the greatest 

 diversities of structure in the several orders and genera. .The whole 

 of this apparatus may be reduced to three principal divisions : 1st. 

 The lingual bone and branchiostegous rays. 2d. The branchial 

 arches. 3d. The opercula or gill-covers. A fourth division of this 

 part of the skeleton may be regarded as formed by the upper and lower 

 pharyngeal bones or maxillae, from the resemblance they bear to 

 branchial arches ; but as they do not support branchiae, but only teeth, 

 they more properly belong to the organs of digestion. 



In the Osseous Fishes the Hyoid bone consists of a large arch, 

 situated behind the lower jaw in front of the first branchial arch, 

 and formed of several bony pieces symmetrically placed upon either 

 side of a single intermediate piece. The lateral branches of the 

 hyoid arch are of very large size in the Osseous Fishes, and corre- 

 spond to the great cornua ; they are composed almost invariably of 

 four pieces, the posterior of which, mostly style-shaped, is the me- 

 dium for attachment of the hyoid to the bone that is analogous to 

 the os quadratum ; the several pieces sometimes coalesce into two 

 or one, but are generally united by fibro-cartilaginous bands. In 

 front the two lateral branches abut against and are united by a single 

 intermediate ossicle (copula) ; this represents the body of the hyoid 

 bone, and commonly supports in front an elongated ossicle, upon 

 which the rudimentary tongue rests ; this is called the lingual bone, 

 and is frequently provided with teeth. To the two branches exter- 

 nally a series of mostly narrow and slender, or frequently broad and 

 strong ossicles, are fastened by ligaments or moveable joints ; they 

 support the opercular membrane of the gills, and have been therefore 

 called the Branchiostegous rays ; their number varies according to th 

 genera and species, or even sometimes the individual. In the true 

 Cartilaginous Fishes, parts occur which correspond to the hyoid arch, 

 but true branchiostegous rays are entirely absent. 



In the above structures we meet with numerous diversities. The 

 hyoid arch is very narrow in Muraena, Syngnathus, and others, and 



