HORNELL— ANATOMY OF PLACUNA 97 



Neph.r). The succeeding serial section on the outer side misses the heart (F) which here 

 is just touched superficiall}\ X 12. 

 Fig. 30. A section belonging to the same series as that of Fig. 29, but nearer the middle line. It 

 shows more clearly the structure of the right nephridium and its posterior csocum, and their 

 relation towards the crystalline style sac and the rectum (i2). As the course of the former 

 is slightly sinuous it appears as two discontinuous portions in this section {CM.s 

 and Csts). X 12. 



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

 {Rei) to show the much folded nature of the wall and the manner in which the left gill 

 (Br) 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 80. 



„ 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 600. 

 Fiars. 35 and 36. Two forms of ova as seen within the duct of the sonad. X 800. 



