POLYCHA4T LARVE. 585 
and those of the other backwards; the cilia of both these 
rows are 80m long. 
The apical pole of the larva bears a circlet of 404 
cilia, and close to this is situated dorsally a slight 
thickening of the body-wall (PI. I., fig. 15, 7’. med.), the 
rudiment of the median cephalic tentacle. Between the 
apical cilia and the prototroch an akrotroch (Pl. L., 
fies. 15, loa, Akr.) extends about half way across the 
ventral surface; of these cilia those near the middle line 
are longer (40) than the rest (20#). 
A little in front of and dorsal to the ends of the 
akrotroch is situated a somewhat crescentic eyespot of an 
extremely dark brown colour. The mouth opens below a 
very prominent upper-lip, along which the prototroch 
extends (Pl. L., figs. 15, 15a). On each side of the mouth 
there extend downwards and forwards from the posterior 
border of the prototroch a row of exceptionally long 
(150) cilia, which are always held straight and motion- 
less, with their distal ends converging (PI. I., fig. 15, C); 
these cilia appear to be absent in some species. Behind 
the prototroch a patch of 30 cila extends backwards 
round the mouth over quite an extensive portion of the 
ventral surface (Pl. I., fig. loa, C. or.). A neurotroch 
(cilia 4m long) extends forwards from the anus, but does 
not quite reach this oral ciliated area. 
The deeply-staining region of the prostomium (rudi- 
ment of the supra-oesophageal ganglion—see Hacker, 
iedo, pp. 20s-9) is large (Pl. I., fic. 15, Ant.). Another 
pair of deeply-staining masses of tissue, meeting in the 
middle line, extend on the ventral surface from the 
prototroch to the posterior end of the body, and mark the 
region of the subsequent development of parapodia and 
ventral nerve cord. The dorsal portion of the body-wall 
is thin and transparent. 
