DEVELOPMENT AND ANATOMY OF SOME EARTHWORMS. 27 
segments ; the whole structure aborts in these segments, so that 
in the adult no nephridia occur in them. 
I find absolutely no trace of provisional nephridia from a 
gastrula stage onwards, though, of course, in a sense all the 
loops which first develop are provisional, and I expect that all 
so-called provisional nephridia will be ultimately explained by 
the fact that the mode of development and ultimate structure, 
and even continued existence of a nephridium, depends upon its 
time of development; and, further, that the nephridia which 
develop early are not confined by septa to their own segments. 
This becomes very clear in large embryos like those of Mahbenus. 
Fig. 41 shows as much of the anatomy of a micronephridium 
of an adult as I have been able to make out; the whole struc- 
ture as drawn is 0°25 mm. long. 
Development of the Nephridia in some other 
Worms. 
In P. pellucida the nephridia develop in the same way as 
in Mahbenus, certain details excepted, as that the funnel 
becomes better developed up to a certain stage, up to such a 
stage as fig. 35. The loop has then the same long wandering 
character. At this stage all resemblance ends. The loop 
becomes broader in proportion to its length, no elongation 
occurs in the neck, no secondary loops are formed, and each 
segment in the adult never possesses more than a single pair of 
nephridia. The five most anterior pairs, after having attained 
a well-developed condition, degenerate and entirely disappear. 
The most anterior pair of nephridia in the adult belong to 
Segment VII. 
The nephridia of Segments vii—x1 undergo further modifi- 
cation until their structure somewhat resembles that of the 
nephridia described by Benham in Microcheta. From a por- 
tion of the ordinary loop a number of outgrowths form, into 
which the tubules run in a very complicated manner, and this 
bunch of outgrowths ultimately form by far the largest portion 
of the nephridium. 
Fig. 42 is taken from a fortunate preparation obtained by 
