170 
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 nephridium, where it usually bears fila- 
mentous blind outgrowths, covered with chlorogogenous 
tissue (figs. 26, 27). The excretory part of the nephri- 
dium is covered with a network of vessels, which le 
between the excretory cells and the ceelomic epithelium. 
This network is well seen in the nephridia of young 
specimens (fig. 29). 
REPRODUCTIVE ORGANS. 
The reproductive organs are closely associated with 
the nephridia. They are found immediately behind each 
nephrostome, except the first, as a small, pinkish, ovoid, 
club-shaped or cylindrical mass of cells, ‘4 to 1:0 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 nephridium, 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, 
2.e., the end in contact with the nephrostome, 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 difterentiated, and in females young oocytes, and 
