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; ma, stomach; 

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

 ovary; h, testes; n, kidney-canal; a/, 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 Bal'anoglos'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 Am'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. 



