PISCES FISHES. 499 



Between the two branches of the os hyoides is placed a single 

 central piece (42), which becomes of great importance in reptiles 

 and birds, and upon this is the bone which supports the tongue, or 

 the Lingual bone (41). 



The great fissure that exists on each side between the head and 

 shoulder of an osseous fish, wherein the gills are situated, is not 

 closed merely by the opercular bones, but likewise by a broad 

 membranous expansion called the Branchiostegous membrane, 

 which is adherent to the os hyoides, and assists in forming the 

 great valve of the operculum. This membrane is supported by a 

 series of slender bones derived from the external margin of each 

 branch of the os hyoides, and these are named from their office the 

 .Branchiostegous Rays (43). 



(536.) Branchial apparatus. Fishes breathe by taking water 

 into their mouths, and forcing it out again through the apertures 

 situated upon each side of the neck ; it is thus made to pass be- 

 tween their gills, which form a series of pectiniform vascular fringes 

 supported upon a system of bones called the Branchial arches. 

 The branchial arches, which are generally four in number on each 

 side, are attached by one extremity to an intermediate chain of 

 bones (53, 54, 55) situated in the mesial line behind the os 

 hyoides, whilst by their opposite extremity they are connected by 

 ligaments to the under surface of the cranium. 



Every branchial arch consists of several pieces (57, 58, 59, 60, 

 61), so joined together by ligaments that the whole is perfectly 

 flexible, and their edges are studded with little osseous plates, 

 generally armed with teeth, and so disposed as to prevent food 

 taken into the mouth from being forced out through the branchial 

 fissures with the issuing streams of water ; so that, in reality, these 

 pieces fulfil in their way the same office as the epiglottis of Mam- 

 malia. 



(537.) Pharyngeal bones.' -The last parts found to enter into 

 the composition of this portion of a fish's skeleton, are called from 

 their position the Pharyngeal bones. They are placed imme- 

 diately behind the branchial apparatus, and form a second set of 

 masticatory organs, generally even more efficient than the jaws 

 themselves, being for the most part provided with very strong 

 teeth. 



In the Perch there are eight of these bones situated just at 

 the entrance to the oesophagus, two inferior (56), and six above 



