TRANSVERSE SECTIONS. . 87 



E. Intestine. 



This occupies the middle of the section. Its diameter 

 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 crescentic 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. 



F. The Nephridia. 



These lie at the sides 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 nephri- 

 dium, on page 76, 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 ccelomic 

 fluid can pass from one segment to another. 



H. The Nerve-cord. 



This lies near the ventral surface, just within the longi- 

 tudinal muscle-layer, and is enclosed in a double connective- 

 tissue sheath, between the two layers of which is a stratum 

 of longitudinal muscle-fibres. 



