572 



CEPHALOCHORDA. 





as a starting-point for the Vertebrate phylum. In most of its organs, 

 Amphioxus exhibits conditions so extraordinarily simple and yet so 

 much in agreement with the fundamental plan of the vertebrate 



ao 



/ .*' 



i 



Fig. 311. — Diagrammatic cross-section through the branchial region of Amphioxus, 

 showing, on the left, the condition of a secondary ami. on the right, that of a 

 primary gill-bar (after Bover] ami Hatschkk). on. aorta ; c, cutis-layer ; ec, 

 endostylar coelom ; /, fascia-layer : fh, dorsal tin-cavity; g, genital vesicle; gl, 

 glomerulus; /.. branchial vessel ; kd, pharynx; Id, ligamentum denticulatum ; m, 

 muscle-plate : mt, transverse muscle : n, renal canal : of, metapleural cavity ; j>, 

 atrial cavity : sc, sub-chordal coelom ; si, truncus arteriosus (branchial artery) : 

 sk, sclera-layer ; »j\ cavities of the lower folds. 



type, that we may well regard it as the " primitive Vertebrate," or 

 at least as a form standing remarkably near the hypothetical ancestor 

 of the Vertebrata. We need here only refer to the primitive con- 



