196 BONY FISHES vn. 3- 



found in other animals is not clear. In the embryo palato-pterygo- 

 quadrate bars and Meckel's cartilages are seen. The upper jaw bears 

 inward projections, which extend towards the chondro-cranium and 

 probably represent the traces of an autostylic means of support (see 

 p. 187). But the effective support in the adult is achieved by the ossi- 

 fied hyomandibular cartilage. The palato-pterygo-quadrate bar ossifies 

 in several parts and palatine, pterygoid, mesopterygoid, metaptery- 

 goid, and quadrate bones appear, some of them partly formed in mem- 

 brane. The only part of Meckel's cartilage to ossify is the articular 

 bone, at the hind end. The actual edges of the jaws are supported by 

 membrane bones, the premaxilla, maxilla, and jugal, covering the 

 upper jaw. The dentary covers most of the lower jaw, except for a 

 small bone, the angular, that lies on the inner side at the posterior end. 



The hyomandibular bone runs from an articulation with the otic 

 capsule to the upper end of the quadrate. The symplectic is a small 

 separate ossification at the lower end of the hyomandibula. The rest 

 of the hyoid arch is present as epi-, cerato-, and hypohyals, which 

 support a large toothed tongue. Bony fishes only rarely possess an 

 open spiracle and immediately behind the hyoid are attached the 

 bones supporting the operculum that covers the gills. The branchial 

 arches are formed of several pieces, as in elasmobranchs, each being 

 ossified separately. 



The effect of this complicated set of bones is to provide an efficient 

 apparatus for the protection of the brain and sense-organs, support 

 of the jaws and teeth and of the respiratory apparatus. Teeth are found 

 on the vomers, palatines, premaxillae, maxillae, dentary, and on the 

 tongue. Covering the typical dentine (orthodentine) is a layer of 

 harder vitrodentine, poor in organic matter and perhaps derived 

 partly from ectodermal ameloblasts. The teeth are usually spikes 

 pointing in a backwards direction, used to prevent the escape of the 

 food and not usually for biting or crushing. They may, however, form 

 plates or be firmly attached to the bones. Folds of the mucous mem- 

 brane, supported by cartilage and carrying gill-rakers, are found in 

 species that feed on small prey. 



4. Respiration 



Limitations are imposed on the respiration of fishes by the facts 

 that water is 800 times more dense than air and the dissolved oxygen 

 is 30 times more dilute. The cost of respiration is therefore high. There 

 is a 70 per cent, increase in metabolism when a trout increases its 

 ventilation volume four times in water poor in oxygen. 



