TEANSVEESE SECTIONS. 79 



E. Intestine. 



This occupies the middle of the section. Its dianaeter 

 is about half that of the body of the worm. 



1. The typhlosole is a fold of the dorsal wall extending 



into the intestine from above, narrowing its cavity, 

 and making it erescentic in transverse section. 



2. The epithelium lining the intestine consists of a single 



layer of columnar ciliated cells. 



3. The muscular layer of the intestinal wall is thin. The 



fibres, which are nearly all transverse in direction, 

 are imbedded in a connective-tissue layer. 



4. The ' hepatic ' peritoneal cells lie outside the muscular 



layer around the vessels which return the blood from 

 the intestine to the dorsal vessel. They are elongated 

 granular cells of large size, and generally arranged 

 radially to the surface of the intestine. They are 

 very abundant in the cavity of the typhlosole, which 

 they almost completely fill. 



F. The Nephridia. 



These lie at the si^es of the alimentary canal. Their 

 appearance in transverse sections varies greatly according 

 to the part of the nephridium through which the section 

 happens to pass. By reference to the description of a 

 nephridium, on page 68, it will be fairly easy to identify the 

 parts present in any particular section that may be examined. 



G. The Septa. 



Owing to their obliquity, parts of the septa are present 

 in nearly all transverse sections, and lie in the body-cavity 

 between the body-wall and the alimentary canal. Each 

 septum is a thin sheet of connective tissue, in which are 

 circular and radial muscles. The ventral edge of the septum 

 is not attached to the body-wall near the middle line, a gap 

 being left around the nerve-cord through which the coelomic 

 fluid can pass from one segment to another. 



