26 ON THE ANATOMY OF MEGASCOLIDES AUSTRALIS, 
The ducts leading to the exterior of the body vary slightly in structure in 
different parts of the body. 
1.—In the clitellar region (Figs. 6 and 28) each duct passes through the 
longitudinal muscle layer, where, close to the ccelom, it may apparently be joined by 
branches from other ducts, and then runs on through the circular fibres. So far it is 
distinctly intra-cellular (Fig. 6, neph.) and is always somewhat coiled, so that the 
duct when cut in section has the characteristic appearance of a large number of 
sections of little ducts so arranged as to form a continuous string (cf. fig. 18.) 
This can be seen most clearly in the clitellar part, where the body wall is thicker. 
The duct on its way through the muscle layers is surrounded by a sheath of 
connective tissue fibres, and is always accompanied by a very distinct blood-vessel. 
(bv.) This vessel may sometimes, as represented by Bzpparp in Acanthodrilus,* 
form a loop round the nephridium, but more generally (as drawn in figs. 6 and 28) 
forms a loop at one side of the duct when the latter is passing through the special 
glandular development, which is characteristic of the clitellum. The intra-cellular 
duct passes just within the layer of gland cells, and there opens into the extra-cellular 
duct leading to the surface. The structure of this part was found to be most clearly 
shown in worms which had been killed by dropping them into a warm saturated 
solution of corrosive sublimate. In the ordinary spirit specimens, the structure, 
though discernible, after having once realised its true nature by means of the better 
preserved ones, is not nearly so clearly visible, owing doubtless to the greater 
contraction of the tissues.: 
The duct is of large size, and owing to its not running in a perfectly straight line, 
but being thrown into a series of small folds, it presents the appearance in section of 
a series of vesicular spaces lying closely one above the other, and often showing 
openings into each other. The walls are extremely fine and thin. In section the 
thin film-like wall of one side of the duct may be often seen (neph., fig. 28) its 
protoplasm, rendered visible by the presence of very fine granules indeed and oval 
nuclei of a large size, containing scattered chromatin fragments. The outlines of the 
cells, which thus form an extremely thin pavement epithelium, are not distinguish- 
able. Sections of the tube show at the edges the nuclei bulging out the thin 
walls (n.) When the surface of the body is reached, a very curious differentiation 
in the epidermis is seen. A certain number of them become so arranged as to form a 
sphere. The different cells composing this are swollen out medianly where the 
nucleus is placed, and taper off towards each end. The two ends are attached 
respectively to the two poles of the sphere, and along the axis of the latter passes a 
tubular cavity opening at the one end to the exterior, and at the other into the 
nephridial duct, the lining cells of which are directly continuous with those lining the 
axial cavity of the sphere. The cuticle does not pass down the duct. 
* Op. cit. Plate 30, fig. 10. 
