HORNELL ANATOMY OF PLACUNA 97 



Neph.r). The succeeding serial section nn (lie outer siile misses (lie heart ( (' ) which hen- 

 is just touched .superficially. X 12. 



Fig. 30. A section belonging to the sunie series as that of Fig. 20, hut nearer the middle line. It 

 shows more clearly tho structure of the right nophridium and its posterior ciecum, and their 

 relation towards tho crystalline style sac and the reel inn (It-. As the course of the former 



>is slightly sinuous it appears as two discontinuous portions in this section " 

 and C.*t.x). x 12. 



31. Transverse section of the left nephridium (Neph.l) at the insertion of the retractor muscle 

 (Ret) to show the much folded nature of the wall and the manner in which the left gill 

 (/?) is suspended by a mesentery from its outer margin. X 20. 

 Note. The individual sectioned was not over four months old ; in adults the foldings of the 



nephridial wall are much more complicated and more numerous. 

 32. Section of the mantle lobe showing the marginal and velar processes and the origin of the 



periostracum (Periost) within a deep narrow groove. X SO. 

 33. Face view of three branchial filaments to show the method of interlocking by means of ciliated 



discs close to the free or ventral edges of the gills. X 120. 



,, 34. A typical gill filament in transverse section to show the histological structure and the arrange- 

 ment of cilia elsewhere than at the level of the ciliated discs. X COO. 

 Figs. 35 and 3G. Two forms of ova as seen within the duct of the gonad. X SOO. 



