620 TRANSACTIONS LIVERPOOL -BIOLOGICAL SOCIETY. 
inserted ventrally in the dorsal sheathing portion of the 
prostomium, was found to be developing in the oldest of 
the Plymouth larvae that were seen. A tentacle in this 
position is found in the Spioniform worm Pocedlochaetus, 
but any close affinity with this genus is rendered 
improbable by the entire absence from the larva of 
Poecilochaetus (Spronid A of this paper)—unless it be in 
the very early stages prior to that in which it first appears 
in the tow-net—of the cephalic structures characteristic 
of the ‘“ Chaetosphaera” larva. At present it is 
impossible to identify the larval genus “ Chaetosphaera ” 
with any known adult genus, but the many points of 
resemblance between this genus and the larvae of Spio 
and Polydora sufficiently prove it to belong to the 
Spioniformia. 
POLYDORID&. 
Polydora.—Metatrochophore (Pl. II., fig. 
31)—A species of Metatrochophore resembling that 
described by Claparéde (1863: pp. 69-70; Pl. VII., figs. 
4-6) as the young of Leucodora, Jnstn. (= Polydora, Bosc.) 
ciliata was obtained on several occasions; in no case, 
however, have I seen a specimen intermediate between 
these larvae and either form of Polydora Nectosoma 
described below. None of these Metatrochophores have 
been examined alive; when fixed they appear to be very 
much contracted. A 5-segment specimen is 300 long, 
its maximum breadth (300) occurring in the body- 
region. The head is large, and almost hemispherical in 
shape; it is 200“ broad posteriorly, and bears the 
rudiments of a pair of tentacles. Claparéde, in his 
5-segment stage, figures a powerful prototroch and 
telotroch, a band of shorter cilia situated on the fifth 
segment, the rudiment of a neurotroch on the first two 
segments, and a pair of short tufts of cilia on the 
