1096 REPORT—1885. 
more post-oral rings (Cheetoped larve, Holothurian larve). We may here assume 
that the ancestral condition was a single circum-oral ring associated with a ter- 
minal mouth and the absence of an anus, and that the existence of other rings 
posterior to this is an expression of metameric segmentation, é.e., a repetition of 
similar parts. With the development of a prostomiate condition a certain change 
necessarily takes place in the position of this band; a portion of it comes to lie 
longitudinally, but it may still remain a single band, as in the larvee of many Echi- 
noderms. How have the other above-mentioned conditions of the velum come. 
about? How has the pre-oral band been developed? Two views have been held 
with regard to this question. According to the one view, the fact whether the 
single band is a pre-oral or a post-oral one depends upon the position in which 
the anus is about to develop. If the anus develops in such a position that 
mouth and anus lie upon one and the same side of the band, the latter becomes pree- 
oral; if, however, the anus develops so that mouth and anus lie upon opposite 
sides of the band, the band becomes post-oral. If we hold this view, we must 
consider any second band, whether pre- or post-oral, to arise as a new development. 
The other view premises that the anus always forms so as to leave the primitive ring 
or ‘architroch’ post-oral, z.e., between mouth and anus, Concurrently with the 
development of a prostomium this architroch somewhat changes its position, and 
the two lateral portions come to lie longitudinally ; these may be supposed to have: 
met in the median dorsal line, and to have coalesced, so as to leave two rings, the 
one pre-oral (a ‘cephalotroch’), the other post-oral (a ‘branchiotroch’). This 
latter may atrophy, leaving the single pre-oral ring, or it may become further 
developed and thrown into more or less elaborate folds. 
The existing condition of the trochal disc or velum in the Rotifera seems to 
the author to bear out the latter view as to the way in which the modifications of . 
the yelum may have come about; further, these results may be well compared 
with those recently obtained by Selenka in the Sipunculids. The trochal dise in 
the Rotifera in its simplest condition forms a single circum-oral ring, as in Micro- 
codon. 'This simple ring may be thrown into folds, so forming a series of processes 
standing up around the mouth; this is the condition in Stephanoceros. There are, 
however, but few forms presenting this simple condition, and it must be remembered 
that the evidence for the assumption here made is at present inconclusive. This 
band may, while remaining single and perfectly continuous, become prolonged 
around a lobe overhanging the mouth—a prostomium, This condition occurs in 
Philodina; the two sides of the post-oral ring do not meet dorsally, but are carried 
up, and are continuous with the row of cilia lining the ‘wheels.’ There is thus 
one continuous ciliated band, a portion which runs up in front of the mouth. 
This condition corresponds to that of the Auricularian larva. The folding of the 
band has become already somewhat complicated. We have only to go a slight step 
further and the prostomial portion of the band becomes separated as a distinct ring, a 
cephalotroch. We find such a stage in Lacinularia, where both cephalotroch and 
branchiotroch remain fairly simple in shape. In Melicerta the branchiotroch is 
becoming thrown into folds. Lastly, we find that in such forms as Brachionus the 
cephalotroch becomes first convoluted and then discontinuous, and further it may 
become so reduced as to be represented only by a few isolated tufts. In such a 
form as Lindia the branchiotroch has become reduced to be two small patches at 
the sides of the head. 
7. On Budding in the Oligocheta. 
By Professor A. G. Bourne, D.Sc., F.D.S. 
The author, while working at the Naidide with the view of preparing a 
monograph upon the group, has been enabled to make a series of observations upon 
the exact method of formation of the bud. The observations here described were 
made upon Nats (Stylaria) proboscidea, and there appears to be some variation in 
the process, as it occurs in different genera or species. In JN. proboscidea, when 
budding is about to commence, a slight thickening of one of the septa which 
separate one ccelomic seyment from another occurs. This thickening increases, 
