74 THE EARTHWORM 



4. The ' hepatic ' 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 ia the cavity of the typhlosole, which 

 they almost completely fill. 



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 

 nephridium, on page 64, 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, lying 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 central part of the ventral 

 edge of the septum is not attached to the body-wall, a gap 

 being left through which the ccelomic fluid can pass from 

 one segment to another. 



H. The Nerve-cord. 



This Hes near the ventral surface, just within the longitu- 

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

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

 of longitudinal muscle-fibres. 



In the sheath lie three longitudinal blood-vessels, a sub- 

 neural below the cord, and a pair of lateral neural vessels at 

 the sides. Along the dorsal surface of the cord are three 

 ' giant-fibres ' of unknown function. The fibrous connective 

 tissue of the sheath extends into the cord, dividing its fibres 

 into two main bundles, and subdividing each of these into 

 smaller ones. Nerve-cells of large size, each with distinct 

 nucleus and nucleolus, form a layer along the whole length of 



