636 PISCES. 



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). 



(1726.) 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 between 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. 



(1727.) 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 Mammalia. 



(1728.) 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 immediately 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. 



(1729.) In the Perch there are eight of these bones, situated just at 

 the entrance to the oesophagus, two inferior (56), and six above (62) : 

 their office and efficiency as organs of mastication must be obvious to 

 the most superficial observer. 



(1730.) Upon reviewing the general disposition of the skeleton in 

 one of the osseous fishes, it is at once apparent that the great instru- 

 ment of locomotion is the tail, which by extensive and vigorous lateral 

 movements sculls the body rapidly along through the yielding element 

 in which these creatures live. In the construction of the caudal ex- 

 tremity of the skeleton, every precaution has evidently been taken to 

 convert this part of the body into a broad and expanded oar, possessed 

 of the utmost possible flexibility in the lateral direction. No pelvis, 

 therefore, trammels the movements of the spine, neither do any trans- 

 verse processes limit the extent of flexion from side to side ; while, on 

 the contrary, the extraordinary development of the spinous processes, 

 both above and below, and more especially the vertical caudal fin, give 

 an extent of surface proportionate to the wants of the animal. 



(1731.) The dorsal and anal fins, situated upon the mesial plane, 

 steady, and perhaps, in some measure, direct the movements of the 

 body ; while the arms and legs, or, rather, the pectoral and ventral fins, 



