DEVELOPMENT OF ACANTHODRILUS MULTIPORUS. 529 
and Wilson (15). I leave out of consideration the larval ex- 
cretory organs, which consist of epiblastic cells in the neigh- 
bourhood of the mouth, known as ‘‘Schluckzellen,” for they 
appear to have no relation to the excretory system of the 
adult. 
In the young embryo there are a pair of provisional excretory 
organs, “ pronephridia”’ they have been termed by Vejdovsky. 
They have also been termed the head kidney. These organs 
appear in very young larve (younger than the earliest stage 
of Acanthodrilus described by me) as a pair of delicate 
ciliated tubes lying on the dorso-lateral walls of the archen- 
teron, and, according to Wilson, who could find neither ex- 
ternal nor internal orifice, actually embedded in the walls of 
the archenteron. Lehmann (21) found no aperture into the 
coelom; the canal runs along nearly the whole length of the 
very young embryo, and opens by a pore at about the middle 
of its length. In later stages only the anterior portion appears 
to be present; it runs from the head-cavity backwards and 
opens on to the 4th segment; in this embryo were three pairs 
of definitive nephridia, The head kidney runs in the body- 
cavity; anteriorly it terminates in a single large cell traversed 
by several canals. 
Vejdovsky’s account is different ; according to him the aper- 
ture is anterior in position, and lies (cf. his tab. xix, fig. 15, 
pn.) Just behind the brain upon the Ist segment of the body; 
therefore it occupies two segments, and the permanent 
nephridia, which develop before the pronephridium disappears, 
only commence in the 3rd segment. The position of the ex- 
ternal pore, it should be observed, does not correspond with 
that of the permanent nephridia. 
In Rhynchelmis, again, there is a pronephridium of the 
same kind on each side of the body, i. e. a single pair which 
occupy the first two or three segments; these, however, are 
figured as opening below and to the side of the cerebral ganglia 
(cf. Vejdovsky’s tab. xii, figs. 12, 13, 15, pr. or pn.). 
In Criodrilus Bergh (12) finds that the pronephridia open 
posteriorly on the sides of the body ; they end blindly in front 
