612 WY, HH, GRAVELY. 
from Port Erin (for a further description of this larva see 
Gravely, 1909) will, I think, throw some light on this point, 
especially when compared with one of Claparéde’s figures 
of the trochophore stage of his Polydora (Leucodora) 
larvee, from which the accompanying trochophore diagram is 
taken (Claparéde, loc. cit.). From the latter it will be 
seen that the ridge bearing the prototroch (which is com- 
plete at this stage as in the typical “trochophore larva’’) 
is already being somewhat pressed aside in the immediate 
neighbourhood of the mouth. This process would appear to 
be continued until a gap is formed in the ridge, the proto- 
TEXT-FIG. 3A. TEXT-FIG. 3B. 
A. Trochophore of Polydora (after Claparéde). 3B. Anterior end of 
Spio larva from Port Erin. Z. Lip. M. Mouth. Ptr. Prototroch. 
Vtb. Vestibule. 
troch disappearing as such in this region and being replaced 
by an area of short cilia. The margins of this gap form the 
lips, whose further growth is mainly in an anterior direction, 
and this brings us to the stage illustrated by the metatrocho- 
phore of Spio. It will be noticed that im this the now 
incomplete prototroch has become somewhat drawn back on 
to the posterior part of the lips; and this, as well as my 
observations upon older larvae, leads me to believe that it 
forms the outer and posterior boundary of the ciliated area 
of the lips. 
By the further enlargement of the lips it is now an easy 
step to the condition found in the typical Nectosoma, where 
