570 CHORDATA 



14. The heart, consisting of auricle and ventricle, lies ventrally in a 

 pericardium. In gill-lireathing species it contains only venous blood, but 

 with pulmonary respiration it is divided into venous and arterial halves. 

 The circulation is closed. 



15. The sexes are usually separate. In most species the excretory 

 (nephridial) system forms the ducts for the reproductive products (urogen- 

 ital system). 



16. The reproduction is strictly sexual. 



17. In the Cyclosto:\uta there is a primitive skull; but vertebra?, 

 paired fins, true scales, and teeth are lacking. The gills are saccular and 

 the nose is unpaired. There is no skeleton to the mouth (no jaws). 



iS. The true fishes (Pisces) have jaws (Gnathostomata). The fishes 

 are further distinguished from the Cyclostomes by the vertebral column 

 (usually amphiccele vertebra;), by paired pectoral and ventral fins, scales, 

 and paired nostrils. They breathe by gills, and have a venous heart with 

 auricle and ventricle. 



19. The fishes are divided into Elasmobranchii, Ganoidei, Teleostei, 

 and Dipnoi. 



20. The Elasmobrandiii have a cartilaginous skeleton, usually a hetero- 

 cercal tail, placoid scales, usualh- a ventral mouth, gills covered, heart 

 with arterial cone, spiral valve in the intestine, no swim bladder. 



21. They are divided into Selachii (subdivided into Squali, sharks, 

 and Raia;, slcates) and Holocephali. 



22. The Teleostei have bony skeleton, usually a homocercal tail, usu- 

 ally cycloid or ctenoid scales, comb-like gills and operculum, bulbus 

 arteriosus, usually pyloric appendages, and a swim bladder; no spiral 

 valve. 



23. They are subdivided into Physostomi, Pharvngognathi, Acan- 

 thopteri, Anacanthini, Lophobranchu, and Plectognathii. 



24. The Ganoidei form a connecting group; they resemble the elas- 

 mobranchs in the presence of a conus arteriosus and spiral valve, and usu- 

 ally in the heterocercal tail; they are like the teleosts in operculum and 

 comb-formed gills, swim bladder, and pyloric appendages. They usually 

 have fulcra and ganoid scales. 



25- The ganoids are di\ided into Chondrostei, with cartilaginous, and 

 Crossopterygii aird Holostei, widi laony skeletons. 



26. The Dipnoi ha\-e gills; occasionally the sv.-im bladder serves as 

 lungs; heart with beginning division; nose with choana. 



27. The Amphibia, in contrast to the fishes, have pentadactyle 

 appendages; in contrast to the reptiles, double occipital condvles. They 

 have bushy external gills and lungs, either persisting together or succeeding 



