530 FRANK E. BEDDARD. - 
in the head-cavity. Otherwise their structure appears to be 
that of the pronephridia of Lumbricus and the other types 
studied. 
The differences between Criodrilus and the other types are 
remarkable, and not at all easy to understand. 
But, apart from this genus, there is considerable evidence in 
favour of regarding the pronephridia as homologous (homo- 
dynamous) with the permanent nephridia. The permanent 
nephridia only commence after the pronephridia. The difference 
in form of the pronephridia may be correlated with their early 
appearance, and with the needs of the larva. The fact that 
they occupy two segments is more puzzling. Vejdovsky explains 
it for Rhynchelmis (1, p. 290) by the incomplete formation 
of the septa at the date of the appearance of the pronephridia ; 
they are thus able to extend further back, being uninter- 
fered with by the formation of the intersegmental partitions. 
In the aquatic Oligochzeta—in many of them, at least— 
a number of pairs of nephridia after the first pair also disappear, 
which is further evidence for regarding the whole series of 
nephridia, including the first pair, as homodynamous. 
In Rhynchelmis the pronephridial stage is marked’ by the 
formation of a closed cell with a single flagellum ; this after- 
wards develops into the funnel; there is no such change in Lum- 
bricus. The pronephridia are directly converted into the 
permanent nephridia. 
The correspondence between the pronephridia of the Ist 
segment (head kidney”’) and the succeeding nephridia is 
strengthened by the observations recorded in the present con- 
tribution. In the youngest embryo at my disposal a pair of 
nephridia lie in the first two segments, and open on to the 
exterior just within the mouth-cavity. These nephridia do not 
degenerate as development advances, but become more com- 
plicated, and remain as the mucous glands. Now the embryo 
in which these nephridia are in the simplest stage of develop- 
ment, resembling in almost every particular the nephridia 
which follow, is apparently not older than embryos of 
Lum bricus with functional head kidneys. In both cases, it 
