190 



BRANCH CHORDATA 



chord is a smooth, elastic rod typically developed from the endo- 

 derm, extending along the median line between the alimentary 

 tube and the central nervous system. It is encased in a tough 

 sheath or membrane and "forms an elastic supporting structure." 



Fig. 152. — Ideal primitive vertebrate, seen from the left side: na, Nose; 

 au, eye; md, mouth; g, ear; ks, gill openings; x, notochord; mr, spinal tube; 

 kg, gill-vessels; k, gill-intestine; hz, heart; ms, muscles; ??«a, stomach; 

 V, intestinal vein; c, body cavity; a, aorta; I, liver; d, small intestine; e, 

 ovary; h, testes; ii, kidney-canal; af, anus; Ih, true or leather skin; oh, 

 outer skin (epidermis); /, skin-fold, acting as a fin. (After Hackel.) 



In the higher forms the notochord is replaced by a segmented 

 cartilaginous or bony vertebral column. 



These three characteristics may not be easily recognized by 

 the beginner as he looks at the worm-like BaVanoglos'sus, the 



Fig. 153. 



-The same in transverse section through the ovaries; lettering as 

 in the preceding figure. (After Hackel.) 



sac-like sea-squirt, or the small fish-like or worm-like A m'phiox'us 

 or Lance'let, but, passing by these low forms to the fishes, frogs, 

 reptiles, birds, and mammals, one readily finds that the body has 

 two cavities instead of one, as in the invertebrates. 



