STUDIES ON POLYCHAT LARVA. 611 
the anterior border of the second segment (see fig. 1, Nér.). 
The stout cilia, too, of which a single pair were noted in 
the vestibule of Spionid A, were here seen to be much 
more numerous, though this may have been due to the 
position of the lips, their complete distribution from end to 
end of the lips (see fig. 1 of a specimen with the lips well 
drawn back) not being always visible even in this species. 
For further details of the ventral surface see Pl. 14, fig. 1. 
On the dorsal surface the differences between this larva and 
Spionid A are perhaps a little more striking. | Nototrochs, 
instead of being entirely absent, are present on every segment 
after the second. As in Spionid A the tentacles arise 
behind the prototroch. The prototroch does not extend com- 
pletely across the dorsal surface (see fig. 2), but no ciliated 
crooves could be found in front of it. Behind the tentacles, on 
the other hand, a pair of extensive ciliated grooves was found ; 
in a 25-segment larva (the oldest examined) these grooves 
commenced on the posterior part of the first sezment near the 
middle line and extended forwards and then outwards, some- 
what in the form of a U, to the sides of the same segment 
slightly behind the bases of the tentacles (fig. 2, C. Gr.) 
The cilia of the prototroch are 80 long; those of the telo- 
troch, which is incomplete dorsally as in Spionid A, 1004 
long. 
The Development of the Vestibule and its Effect 
upon other Organs. 
Although it would appear to be characteristic of the necto- 
soma stage of Spioniform larvee to bear a pair of large lateral 
hps closing in a “vestibule” in front of the true mouth, 
these are not present in the earliest stages ; they are absent, 
for instance, in the metatrochophore of Spionid C of the 
Port Hrin larvee, a species of which, unfortunately, no other 
stage is yet known. I have not been able to study their 
development at all fully, but the accompanying diagram of 
the anterior end of a metatrochophore of a species of Spio 
VOL. 53, PART 3.—NEW SERIES. A2 
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