606 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
in length, and often appear in the trochophore stage. 
The segmented larvae are in general much more slender 
than those of the Nereidiformia, and the characteristic 
pair of tentacles usually begin to develop immediately 
behind the prototroch, at an early stage. 
SPIONID. 
Spio. (Pl. III., fig. 38) —Metatrocho phore== 
The youngest larvae seen are of a small size (300p by 
100), and bear three pairs of very conspicuous black 
eye-spots that are very resistant to the solvent action of 
spirit. The eyes may be arranged in a straight line with 
the large, somewhat crescent-shaped pair at the sides of 
the head and the other two pairs nearer the middle-line 
on the dorsal surface; or their position may, in the same 
specimen, change so as to present the arrangement shown 
in Pl. III., fig. 58, or some intermediate pattern. 
Each segment bears a tuft of exceedingly long and 
slightly curved setae, which are held parallel to the sides 
of the body when the larva is swimming, but when it is 
irritated they project from the body as shown in Pl. ITT., 
fie. 38. These setae are minutely serrate on the convex 
side, but the serrations are quite indistinguishable in a 
balsam mount. As the larva grows they increase in 
length also, so that the longest ones are always found to 
be about twice the length of the body. The oldest larva 
seen has eight seements, and is 550 in length. 
The observations upon this larva during life are not 
sufficiently complete to allow of the structures then seen 
being adequately figured; but apical cilia and (incom- 
plete ?) interparatrochs are certainly present. The proto- 
troch seems to be invariably composed of shorter cilia 
(40) than the telotroch (504). The course of the proto- 
troch is interrupted in the middle of the dorsal surface 
