250 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



angle, where it is usually joined by the vessel which has 

 traversed the ventral lip. The gonidial vessel formed by 

 their union runs over the excretory part of the organ, and 

 sends branches to the vesicle, finally opening into the 

 nephridial longitudinal vessel. In the upper part of its 

 course the gonidial vessel is surrounded by the gonad, 

 except in the first nephridiuni, where it usually bears fila- 

 mentous blind outgrowths, covered with chlorogogeiious 

 tissue (figs. 26, 27). The excretory part of the nephri- 

 diuni is covered with a network of vessels, which lie 

 between the excretory cells and the coelomic epithelium. 

 This network is well seen in the nephridia of young 

 specimens (fig. 25). 



Reproductive Organs. 



The reproductive organs are closely associated with 

 the nephridia. They are found immediately behind each 

 nephrostomy, except the first, as a small, pinkish, ovoid, 

 club-shaped or cylindrical mass of cells, '4 to TO mm. long, 

 surrounding the gonidial vessel and apparently produced 

 by proliferation of its cell covering (fig. 24). The gonidial 

 vessel is developed on the nephridia in very early life, it 

 may be recognised even in post-larval stages. In a 

 specimen 44 mm. long the gonads, though minute, are 

 recognisable (fig. 25). The gonad is well seen in an 

 adult stained nephridiuni, being distinguished by its 

 affinity for stains {e.g., carmine or hematoxylin). It is a 

 closely packed mass of cells in which at the anterior end, 

 i.e., the end in contact with the nephrostomy, the cells are 

 small, almost uniform in size, and have well-marked, 

 deeply-staining nuclei. In the middle and posterior 

 portions of its length the cells on the surface of the gonad 

 become differentiated, and in females young oocytes, and 



