Fy HENRY BURY. 
and veutral surfaces ; and in their delicacy, which renders it 
almost an impossibility to preserve and embed them without 
losing much of the original form by shrinkage. I have suc- 
ceeded fairly well by carefully embedding in celloidin, but 
the tissues are never so clear as in the living anima]. Almost 
equally good results, with greater economy of material, may 
be obtained by balancing the living larva in a watch-glass 
with one hand, making a rough sketch with the other, and 
gradually correcting and adding details with the help of 
repeated observations. No doubt this requires practice, but 
it has the advantage that several stages can be followed on the 
same larva. All my polar diagrams (figs. 6—9) were made 
in this way, and though not drawn quite to the same scale, 
are accurate enough for my present purpose. 
Semon has drawn attention to the marked diminution in 
size which accompanies metamorphosis, and has given some 
measurements (32, p. 29), but these can only be regarded as 
approximate, the variations in size being great. 
The discrepancies between Metschnikoff’s account and 
Semon’s are so very large, that I naturally looked out carefully 
for any abnormalities which might help to reconcile them ; 
but though a good deal of variation was noticeable (some 
of it possibly due to specific differences), none of it 
threw any light on this point. Metschnikoff’s description 
and figures are on the whole extremely accurate, and my 
account is rather an addition to his than a correction of it. 
It is curious, however, that he overlooked the asymmetrical 
position of the mouth and atrium, since two of his figures show 
it (21, pl. ii, figs. 16 and 18), one of them representing the 
water-pore and atrium on the same side of the body, which 
could not happen if they occupied the positions he assigns to 
them. His failure to understand the formation of the two 
anterior ciliated rings was probably due partly to the difficulty 
of obtaining polar views, which alone render the changes 
intelligible ; and partly to his having missed that stage, which, 
in my experience, usually occurs at night. 
Semon’s account I am wholly at a loss to understand, so 
