12 
ON A NEW SPECIES OF ENTEROPNEUSTA, 
transversely outwards, reducing the lateral walls of the blind sac 
to a thin layer, while the hind and middle portions of the wall 
are somewhat thicker, as in Pt. minuta. From the transverse 
lumen of the blind sac there passes forwards laterally a short horn 
on each side (fig. 9, lb.); a section passing through the proboscis 
neck just anterior to the passing down of the ventral blind sac 
lumen thus shows three cavities in the notochord (fig. 9), two ventro- 
lateral belonging to the horns of the ventral blind sac lumen and 
a median situated near to the dorsal side of the notochord, the 
main notochordal lumen now considerably reduced in size and with 
numerous gland cells opening into it. Anteriorly the lumen 
comes to occupy a more nearly central position (fig. 8, div.); it 
finally becomes reduced to a narrow slit (fig. 4, div.), which can 
be traced to a slightly varying distance from the apex of the 
notochord (fig. 14, div.). The lumen varies somewhat in position 
and shape in different individuals, being in some situated nearer 
the dorsal side, in others nearer the ventral side of the notochord; 
its outline also varies — it may be circular or form a narrow 
vertical or horizontal cleft. The lumen in this species has on the 
whole a more median position than in Pt. minuta, and con- 
sequently the dorsal and ventral walls are more nearly equal in 
thickness. In fig. 14 the lumen of the notochord is represented 
somewhat diagrammatically as a continuous wide canal, but as in 
Pt. minuta it is interrupted by occasional bridges of tissue, and 
also as in that species the dorsal wall of the lumen is smooth, 
while into the ventral there project short processes of the lumen 
(fig. 14, div.). With the exception of the anterior part of the 
lumen numerous gland cells open into it, testifying to the epithe- 
lial nature of the whole structure. Numerous gland cells exist in 
the dorsal wall of the neck portion of the notochord and they 
extend for a considerable distance into the head region, being 
specially abundant round the part of the lumen situated just 
anterior to the point of origin of the ventral blind sac lumen. 
The lumen is, in some preparations, almost filled up by a diffusely 
staining network, apparently derived from the secretion of the 
gland cells opening into it. Numerous elongated cells, between 
