STUDIES ON POLYCHAT LARVA. 601 
series of ventral denticulations at the anterior end of the 
pharynx, but no strong tooth or similar dorsal denticulations. 
The present species is closely allied to O. gibba (Claparéde, 
1863, pp. 81-83; Pl. XII, figs. 7-14). It differs therefrom, 
however, in that special natatory setz are present on the para- 
podia of the eighth or ninth and succeeding segments in both 
sexes, instead of from the seventh in the male, and from a 
segment further back (eleventh?) in the female (De Saint- 
Joseph, 1886, p. 174) ; these setze, moreover, would appear to 
be present throughout life in the present species (see below), 
instead of at maturity onlyasin O. gibba. DeSaint-Joseph 
also notes a single capillary seta in each ventral bunch of the 
posterior segments of his specimens of O. gibba, but I 
have been unable to discover this in mature specimens of the 
present form, although in the young specimen it occurs in 
every tuft throughout the whole length of the body. Lastly, 
in this form it is the peristomial, not the first chetigerous, 
segment that is produced forwards dorsally in contracted 
specimens at least, so as to overlap the posterior part of the 
head. 
The jointed setz closely resemble those of O. gibba 
(Claparéde, 1865, Pl. XII, fig. 74), though their proximal seg- 
ment terminates in a somewhat sharper point (text-fig. 2), 
and the shortness of the tentacles and cirri and the presence 
of six denticles is quite characteristic of O. gibba. I have 
been unable up to the present to find any description of a 
species differing from it only in the above-mentioned points. 
Only one specimen of this worm was obtained in a really 
young condition, and I was unable to examine this alive. — It 
was quite small, being only 1°75 mm. long, and possessing 
but twenty-two segments, each about 0°6 mm. broad (exclu- 
sive of appendages). ‘The adult worm may have as many 
as fifty segments, 2 mm. in breadth, the whole being 15 mm. 
long. 
The anterior end of the young specimen is rounded, but 
consists in reality of a pair of large, closely apposed palps, 
projecting in front of the transversely flattened head; 
