CHAPTER III 
ISOSPONDYLI 
still remain for discussion constitute the great sub- 
y| class or series of Teleostei (reAeos, true; ooréor, bone), 
or ar fishes. They lack wholly or partly the Ganoid traits, 
or show them only in the embryo. The tail is slightly, if at all, 
heterocercal; the actinosts of the pectoral fins are few and large, 
rarely over five in number, except among the eels; the fulcra 
disappear; the air-bladder is no longer cellular, except in very 
tare cases, nor does it assist in respiration. The optic nerves 
are separate, one running to each eye without crossing; the 
skeleton is almost entirely bony, the notochord usually dis- 
appearing entirely with age; the valves in the arterial bulb 
are reduced in number, and the spiral valve of the intestines 
disappears. Traces of each of the Ganoid traits may persist 
somewhere in some group, but as a whole we see a distinct 
specialization and a distinct movement toward the fish type, 
with the loss of characters distinctive of sharks, Dipnoans, and 
Ganoids. In a general way the skeleton of all Teleosts corre- 
sponds with that of the striped bass (see Figs 22, 23, Vol. I), and 
the visceral anatomy is in all cases sufficiently like that of the 
sunfish (Fig. 16, Vol. I). 
The mesocoracoid or preecoracoid arch, found in all Ganoids, 
persists in the less specialized types of bony fishes, although 
no trace of it is found in the perch-like forms. With all this, 
there is developed among the bony fishes an infinite variety 
in details of structure. For this reason the Teleostei must be 
broken into many orders, and these orders are very different 
in value and in degrees of distinctness, the various groups being 
joined by numerous and puzzling intergradations. 
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