512 FRANK E. BEDDARD. 
to bring forward this structure as further evidence of a close 
affinity between the Polychzta and the Oligocheta ; but the 
fact does appear to favour the idea of, at any rate, the former 
existence of a free-swimming larval stage among the Oligo- 
cheta, though not necessarily to be found in any living genus. 
It is true that the development of very few genera of this 
family is known; but the formation of a cocoon is apparently 
so unusual that we may suppose it to be a very old character- 
istic of the group, possibly so old as to embrace the ancestors 
of both Oligocheta and Hirudinea. The enclosure of eggs 
within a cocoon does not, it may be admitted, seem favorable 
to the idea of a free-swimming larva. In only one existing 
group is there any evidence of a free-swimming Oligochetous 
larva. Lankester has described and figured (19, p. 642, pl. xlviui, 
fig. 4) the young of Acolosoma quaternarium, which is 
somewhat unlike the adult, and is more extensively ciliated. 
Vejdovsky (2, p. 165) has remarked upon the resemblance of 
this larva to that of a Cheetopod. 
There are several species of Acanthodrilus which are 
aquatic—for instance, “ Mandane stagnalis” of Kinberg; 
but I do not wish to urge the possibility even of there being a 
free-swimming larva in any of these. All that I desire to 
point out is that, so far as we at present know, the develop- 
ment of the group Acanthodrilus is the only type in which 
the traces of a formerly existent (?) free-swimming stage are 
to be found. This is so far an argument for the conclusion that 
Acanthodrilus comes nearer to the ancestral Oligochete 
than such a type as Lumbricus does; other facts appear to 
me to point unmistakably towards the same conclusion. 
§ Stomodeum and Proctodeum. 
In the youngest embryo the stomodeal invagination is already 
fully established, but it does not open into the mesenteron ; 
the opening is formed in the next stage in an embryo hardly 
larger than the youngest, but the aperture of communication 
(see fig. 24) is narrow. The narrow aperture between the 
stomodzum and the mesenteron is still further reduced by a 
