CHORDATA. 



275 



289. Some Doubtful Forms: a Library Exercise. There 

 are some lowly animals that stand in some respects inter- 

 mediate between the invertebrates and the vertebrates. 

 They have some of the qualities of the vertebrates men- 



FIG. 106. Cross section of a vertebrate, showing the arrangement of the principal 

 organs. Diagrammatic, ap, paired appendages; b.c., body cavity; ch, notochord; 

 d.f., dorsal fin; g, gut; h, heart; h. a., haemal arch (bony); m, muscles; n.a., neural arch 

 (bony); sp, spinal cord (nervous); v.c., vertebral column. 



Questions on the Figure. Compare this figure carefully with 

 Figures 6 and 57 (sections of invertebrates), and enumerate the 

 principal differences in the arrangement of the important organs. 

 Study the figure as an illustration of bilateral symmetry. Compare 

 it with the cross-section of the sea-anemone (Fig. 51). 





