608 F, H; GRAVELY. 
make room for this enlargement, the neurotroch is sunk in a 
well-defined groove or pit (see fig. 5). From this a gastro- 
troch, somewhat broken as shown in the figure, extends 
across the segment on each side. 
On the remaining segments there is no neurotroch. Onthe 
third segment the gastrotroch is intermediate in form between 
that of the second and those of the remaining segments. 
On the fourth and succeeding segments it consists on each 
side of an outer and middle row of 40, cilia, and an inner 
row of much shorter and more delicate ones. The gastro- 
trochs become smaller and finally disappear towards the 
posterior unsegmented region of the larva. The anal seg- 
ment, however, bears a powerful telotroch, though this does 
not completely encircle the body, there being a short median 
dorsal gap in its continuity. 
No cilia are present on the dorsal surface of the intertrochal 
segments. The prototroch, whose continuity is interrupted 
ventrally by the vestibule, shows a considerable dorsal gap 
also. Its cilia are 60, long, and are borne on a pair of 
rounded ridges that extend dorsally from the sides of the 
head in a posterior direction (fig. 4, R. Pr.) ; the tentacles, 
when they appear, are situated immediately behind these 
ridges. The inner and anterior edge of each of the ridges 
bounds one side of a groove, which opens out anteriorly and 
contains a row of 20 w cilia (fig. 4, C. Gr.). This groove is 
bounded on the other side by the broad median crest of the 
head. ‘Two pairs of red eye-spots are present; the posterior 
pair are situated upon this crest, and the anterior pair upon 
slight lateral prominences a little anterior to the ciliated 
grooves. ‘These prominences each bear a little tuft of cilia 
(fig. 4, 7. C.) 20 w long, just in front of the anterior pair of eyes. 
In the most advanced specimen examined all cilia had disap- 
peared from the ventral surface of the head (see Gravely, 1909). 
Spionid D. 
This larva, probably identical with Claparéde’s “‘ Larve 
mit riisselartiger Oberlippe” (Claparéde, 1865; foot- 
