594 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
the pharynx becomes first clearly recognisable at this 
time; it is formed from a pair of pocket-like outgrowths 
of the oesophagus, as in Polynoé, but these only appear 
as thickenings of the wall of the oesophagus in whole 
mounts, and can rarely be seen at all until after the 
trochophore stage. The oldest larva obtained is in the 
fixed and shghtly contracted condition 8004 long by 
150 average breadth; twelve intertrochal segments are 
present—their appendages bear slightly larger cirri than 
did those described in the previous stage. The dorsal 
cirrl are more conical than foliaceous, but are stouter 
than the ventral. The setae are all jointed, and of the 
type shown in Pl. II., fig. 7a; they are extremely 
transparent in balsam, and their exact shape is therefore 
very difficult to determine. The prototroch is still 
present, but the akrotroch and oral cilia, and probably 
also the interparatrochs, have disappeared. Only a single © 
pair of eyes is present in this specimen; these are situated 
upon the dorsal surface of the head, and are dark brown 
and very opaque; by very strong transmitted light they 
are seen to be of a somewhat purplish brown colour. The 
pharynx is clearly marked, and joins the intestine 
between the fifth and sixth segments. 
The four pairs of tentacles and three pairs of 
tentacular cirri show that this species belongs to the 
genus Mystrdes, Théel; whilst the arrangement of the 
tentacular cirri on the first two segments only 1s 
characteristic of the sub-genus Mesomystides, Cziernavsky. 
It is thus separated from Mystides lizziae (the only species 
of Mystides recorded by McIntosh in his Monograph of 
British Annelids) which belongs to the sub-genus 
Promystides, with one pair of tentacular cirri on each of 
the first three segments. 
