80 THE EARTHWORM. 



li. The Nerve-cord. 



This lies 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 ves.sel8 at 

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

 ' giant-fibres ' probably of nervous nature. The fibrous con- 

 nective 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 the ventral and lateral surfaces of the cord : they 

 are more abundant in the ganglionic enlargements. 



I. The Blood-vessels. 



These are readily recognised by the deeply stained blood- 

 clots with which they are filled. The larger vessels have 

 muscular walls. Besides the neural vessels just mentioned 

 the following may be recognised : — 



1. The dorsal vessel lies in the body-cavity above the intes- 



tine. It is of large size, and has masses of ' hepatic ' 

 cells at its sides. 



2. The intestinal vessels lie in the walls of the intestine : 



they are surrounded by ' hepatic ' cells, and may in 

 some sections be seen to open into the dorsal vessel. 



3. The ventral vessel lies between the nerve-cord and the 



intestine, and is connected with the latter by a 

 median sheet of vascular connective tissue. 



4. The nephridial vessels. 



5. The capillaries of the skin. 



