KEOENT WORK ON THE PEOTOOHORDA. 



233 



If Eoveii's lubules open the way to a perception of the subse- 

 quent potentialities of the excretory system, Lankester's brown 

 funnels, Hatschek's nephridium, and the prseoral pit furnish a 

 clue to its past history. 



vn 



Fig. 2, A and B. — Portions of transverse sections tbrough the caudal region 

 of Ptychodera ruficollis, n. sp. A. Through the anterior caudal 

 region. B. Through the mid-caudal region, cm. Circular muscles of 

 body-wall. hg. Wall of hind gut. Im. Longitudinal muscles, py. 

 Pygochord. vn. Ventral nerve-cord. vv. Ventral blood-vessel. 



The substitution of nephric tubules in the truncal region 

 for regional pores in the archimeric (Masterman) regions, 

 which is displayed before our eyes in Amphioxus, is one of 

 the most striking examples of the working of the principle of 

 substitution that I can call to mind, 



