CHORDATA 283 



the caudal may be diphycercal, heterocercal, homocercal, or 

 absent. Typically there are both pectoral and pelvic paired 

 tins, and the latter are generally a considerable distance in front 

 of the anus ; they are not provided with claspers. The mouth 

 is most frequently at the anterior extremity of the head, but it 

 may be ventral and sometimes even dorsal. The five gill clefts 

 never open directly to the outside, as in the Elasmobranchii, 

 but are always covered by a fold of the skin, the operculum, 

 which is supported by bony plates ; there is thus left a single 

 slitlike opening on each side of the head. There is no cloacal 

 opening, but a distinct anus, and posterior to it the opening of 

 the urogenital ducts. 



The mouth is usually provided with teeth, sharp and recurved, 

 and attached, not only to the jawbones, but to various bones on 

 the roof of the mouth as well. There is a small tongue attached 

 to the floor of the mouth. In some Teleostomi a spiral valve 

 occurs in the intestine ; in others there are tubular processes at 

 the posterior end of the stomach, called the pyloric caeca, which 

 vary in number from a single one up to two hundred. Fre- 

 quently there lies just dorsal to the body-cavity an elongated 

 sac, the air bladder or swimming bladder, which is sometimes a 

 closed, gas-filled sac, and sometimes connects by a duct with the 

 anterior part of the alimentary canal. It generally serves to 

 facilitate the movement of the fish vertically through the water, 

 but in some cases it may be respiratory. The kidneys lie just 

 dorsal to the air bladder and ventral to the vertebral column. 

 The reproductive organs are in the body-cavity ; the sexes are 

 separate as a rule, although there is one family hermaphroditic 

 and self-fertilizing. Generally the Teleostomi are oviparous, 

 but a few are viviparous. They may be divided into two 

 groups, which for our present purpose may be called orders. 



Order 1. Ganoidei 



The Ganoidei (Gr. 7^0?, brightness, and etSo?, appearance) 

 are extensively represented among fossil fishes, but have only 

 a few living species. They are essentially fresh-water fishes, 

 although some are marine and ascend the rivers to spawn ; the 

 most common living representatives are the sturgeons and the 



