STUDIES ON POLYCHAT LARVA. 609 
note to p. 86; Pl. VII, figs. 1, 2), shows a remarkable modi- 
fication of the above-mentioned lips, and will require a 
separate description. The prototroch is situated but a short 
distance in front of the mouth, the greater part of the lips 
lying in front of it instead of behind as in Spionid A. 
At the anterior end the lips meet in the middle line and 
are prolonged into a slender snout sheathed in a similar pro- 
longation of the prostomium. Under the stimulus of fixing 
reagents these snout-like structures are usually both retracted 
to such an extent as to be almost unrecognisable even in 
sections; however, I think it is almost certain that the 
anterior extensions of the lips are actually fused together in 
the middle line} and also, until very close to their termina- 
tion, at least, fused to the prostomium. But from their 
appearance during life I am led to believe that the “ snout ” 
is free from the prostomium at the extreme anterior end. 
Whether it is pierced by a canal, as seemed probable from 
the examination of the living organism, I have been unable 
to determine on sections. 
This snout is supported by a pair of muscle-bands, very 
conspicuous during life, which pass backwards and outwards 
in the substance of the lips towards the bases of tentacles, 
and are seen in sections to be continuous with a pair of 
muscle-bands that extend back through the whole length of 
the body. Near the bases of the tentacles—which arise 
at rather an early stage in the development of this larva— 
these muscle-bands pass dorsal to a pair of contorted (?)-tubes, 
or masses of (?)-tubes, that are also most clearly seen in the 
living specimen. 
The vestibule is ciliated throughout as in Spionid A, but 
the cilia on the external surface of the lips are confined to an 
area (see fig. 3) which does not extend to their posterior 
margin nor on to their anterior prolongation. 
The neurotroch, as in Spionid A, is confined to the first 
two segments, but on the second it consists of a distinct 
anterior and posterior part separated by a region devoid of 
cilia ; for this and other modifications of the ciliation of the 
