828 



A. T. MASTEBMAN. 



to the new animal apexj and with it the radial nerve-tracts 

 are altered somewhat in position, as also are the coelomic 

 pouches, in correlation to the forward motion of the sub- 

 neural sinus. 



In the next stage, Stage iii (woodcuts 12, 13, and 14), further 



The nervous system is somewhat 



13. 



changes are indicated 



14. 



12. Ventral view of Stage III. 13. Dorsal view of ditto. 14. Lateral 



view of ditto. 



altered. The nerve-ganglion has still progressed forwards, 

 and it is seen that the longitudinal nerve-ring and radial 

 nerves are now tending to form a secondary system of rings 

 in planes at right angles to the axis of motion. Thus two 

 radial nerves and part of the primitive ring form the " pre- 

 oral ring/^ two radial nerves form the '' post-oral ring," and 

 part of the primitive ring forms the " perianal ring." In 

 other words, these rings are formed to correspond to the three 

 archimeric segments — protomeric, mesomeric, and metameric. 



