THE CHORDATES 



389 



Dorsal root of nerve 



Fin ra-y 

 Skin. y 



Nerve cord /^^ 



Ventral root LI 



of ne-Tve 



Dorsal a.orta. 



Protonephriciium 



Epipharyngeal 

 6roove 



Coelom 

 Middut caecum 

 Ovary 



Gill slit 



Gill bar 



Branchial arteri/ 



Endostyle 



Coelom 



omcre 



NotocKord 



Ectodevmai 

 epith-elitLm. 



PtiaJ^yrvjc 



Transverse 

 mtL^cie 



'LynrpK spa.CC 



Ventral ^ \vLetapL^.^ ^. x ^.^ 

 aorta \ J 



Atrium. 



Figure 20.4. A diagrammatic cross section through the posterior part of the 

 pharynx of Ainpliioxiis. Branchial arteries extend from the ventral aorta through the 

 gill bars to the dorsal aortas. The portion of the coelom ventral to the endostyle is 

 connected through alternate gill bars with the pair of coelomic canals lying dorsal 

 to the atrium. Other parts of the coelom are associated with the midgut caecum and 

 gonads. 



gametes are discharged into the atrium upon the rupture of the gonad 

 walls. Fertilization and development are external. 



177. Subphylum Vertebrata 



The Vertebrata is by far the largest and most important of the chor- 

 date subphyla, for all but about 2000 of the approximately 35,000 living 

 species of chordates are vertebrates. The subphylum in turn, is divided 

 into eight classes. The oldest and most primitive vertebrates, which 

 lack jaws and paired appendages, are placed in the class Agnatha. 

 Most of these are extinct, but the lamprey is a living representative of this 

 group. The Agnatha gave rise to the class Placodermi, a group of prim- 

 itive jawed fishes, all of which are extinct. Placoderms, in turn, gave 

 rise to the large groups of living fishes— the class Chondrichthyes and 

 the class Osteichthyes. The Chondrichthyes are the fishes with carti- 

 laginous skeletons such as the sharks and rays; the Osteichthyes are the 

 more familiar fishes with bony skeletons such as salmon, minnows and 

 perch. The first terrestrial vertebrates evolved from certain of the bony 



