572 



CEPHALOCHOKDA. 



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 



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

 showing, on the left, the condition of a secondary and, on the right, that of a 

 primary gill-bar (after BOVERI and HATSCHEK). ao, aorta ; c, cutis-layer ; ec, 

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

 glomerulus ; k, branchial vessel ; kd, pharynx ; Id, ligamentum denticulatuin ; in, 

 muscle-plate ; mt, transverse muscle ; n, renal canal ; of, metapleural cavity ; p, 

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

 sk, sclera-layer ; uf, 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- 



