CLASS PISCES (FISHES). 



Aquatic vertebrates breathing by gills. Heart, as a rule, of two 

 chambers ; blood cold. Two pairs of limbs in the form of paired 

 fins supported by fin -rays are usually present, and are not divided 

 externally into arm, fore-arm and hand, or thigh, leg and foot. 

 They are the only animals possessing median fins supported by 

 fin-rays. A row of sense-organs along each side of the body. Skin 

 generally covered with scales. No amnion and no allantois. 



SUB-CLASS I. TELEOSTEI (BONY FISHES). 



Skeleton bony, with well-developed skull. Tail externally 

 symmetrical (though it may not be so structurally). Brain with 

 large optic lobes ; optic nerves simply cross olfactory organ double. 

 Gills free, in one chamber, and with one aperture of exit behind the 

 ^'ill-cover. Heart with a non-contractile bulb at the origin of the 



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aorta. No spiral valve in the intestine, which opens independently 

 of the urinary and genital apertures. Air-bladder usually present. 

 Eggs numerous, generally small, almost always fertilized in the 

 water. 



ORDER I. 



ACANTHOPTERI. 



Fishes generally having ctenoid scales. Ventral fins thoracic or 

 jugular. Some of the fin- rays spinous, that is, unarticulated. No 

 duct to the air-bladder (physociistous). 



