472 Scientific Proceedings, Royal Dublin Society. 



EXPLANATION OF PLATE X. 



Diagrams Illustrating the Probable Evolution of the Segmental 

 (Archinephric) Duct. 



a., primitive anus, or urodseum (blastopore) ; al., alimentary canal ; 

 ao., dorsal aorta; cce., coalom ; e.o., external orifice of nephridium ; 

 gl., glomerulus of archinephros ; gr., nephric groove ; i.o., internal 

 (ccelomic) ciliated orifice of nephridium ; pr., proctodeum (epi- 

 blastic cloaca) ; s.d., segmental (archinephric) duct. 



Fig. 1. — Horizontal view of the arrangement of the nephridia in Seg- 

 mented Worms. 



,, 2. — Transverse section through the body of an Earthworm (Dim- 

 bricus). 



,, 3. — Transverse section through the trunk of a hypothetical 

 primitive representative of the Chordata. 



,, 4. — End view of the same, to show the anus lying within the 

 nephric groove. 



„ 5. — Horizontal view of probable disposition of the nephridia of the 

 same. 



,, 6. — Horizontal view of ideal archinephros of the lower Verte- 

 brates. 



