318 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



it are covered with c'hlorogogen cells. (See fig. 43.) The 

 nerve cord is divisible into a dorsal fibrous mass, in the 

 mid-dorsal line of which one giant-fibre is seen, and a 

 ventral portion composed largely of nerve cells the nuclei 

 of which are indicated. x 38. 



Fig. 36. Transverse section of the same specimen in 

 the region of the last nephridium. The portion to the 

 right of the two vertical lines is from a section passing 

 through the anterior part of the ninth chsetigerous seg- 

 ment, while that on the left is from a section passing 

 through the tenth parapodium, and showing the notopo- 

 dium with its setal sac and protractor muscles and the 

 whole length of the neuropodium. We see also a portion 

 of a gill, with its afferent and efferent vessels, the funnel 

 (opening into the ccelom) and excretory part of the nephri- 

 dium, the stomach with its ventral groove and blood 

 sinuses, the subintestinal and the lateral gastric, the dorsal 

 and ventral vessels, the latter surrounded by chlorogogen 

 cells (fig. 43), the muscles of the body wall and their blood- 

 vessels ; two oblique muscles ; the nerve cord, the dorsal 

 fibrous mass of which contains two giant-fibres, and the 

 ventral portion with nerve cells, the nuclei of which are 

 shown. x 38. 



Plate V. 



Fig. 37. Longitudinal section of the heart of a young 

 Arenicola 65 mm. long to show the heart body in an early 

 stage of its formation. On the posterior (right) side and 

 also on the antero-external side (left of the figure), the 

 wall of the heart is being invaginated at several points, 

 thus giving rise to the heart body. x 100. 



Fig. 38. A portion of the wall of the ventricle from 

 the point marked + in the preceding figure, and composed 

 of three layers, an endothelium internally, a muscle layer 



