DEVELOPMENT OF ACANTHODRILUS MULTIPORUS. 505 
The nephridia are considerably advanced as compared with 
those of the earlier stages, but are still very far from showing 
the characteristics of those of the adult Acanthodrilus. 
The first pair occupy the first four segments, which corre- 
spond to the extent of the stomodeal invagination; these 
segments are divided by irregularly running septa, and are 
only recognisable by the sete. Apparently, therefore, there is 
an important difference between this stage and the last in 
respect of the number of segments occupied by the “ mucous 
gland.” This difference is, however, only apparent; it is 
caused by the concrescence of the “ mucous gland” with 
the following nephridium. I have carefully followed out 
the first and second pairs of nephridia, and I can nowhere find 
a break of continuity between them, such as is quite obvious 
in the two earlier stages. The original distinctness of 
the two nephridia is still shown by the presence of 
two funnels, occupying precisely the position of the earlier 
stages. I detected the external orifice belonging to the pos- 
terior nephridium ; the whole organ also opens anteriorly as 
in the earlier stages, but in this stage much further within the 
buccal cavity. Moreover the actual orifice is now formed by 
an outgrowth of a tube from the buccal cavity lined with 
columnar cells, continuous with those which form the lining 
of that cavity. The shifting backwards of the aperture of 
the nephridium appears to be related to the shifting back- 
wards of the cerebral ganglia and of the circumcesopha- 
geal commissures ; the point of opening and the position of 
those commissures are close together, opposite to the sete of 
the 2nd segment. The first nephridium extends back beyond 
the third seta, and therefore appears to occupy four segments. 
Fig. 16 represents a portion of two segments of an em- 
bryo at this stage, showing the body-wall, the ventral blood- 
vessel, and the intersegmental septa. The nephridia belonging 
to the segment are not shown in their entirety ; only the funnel 
and the proximal end of a tubule are indicated. The rest of 
the nephridium forms a compact and dense coil of tubules, with 
very thin walls and a comparatively wide lumen. Fig. 2 illus- 
VOL, XXXIII, PART IV.—NEW SER. M M 
