NOTES ON THE STRUCTURE OF OIKOPLEURA. AT 
distance beyond that. It contains in its centre the noto- 
chord (or urochord, 2. ch.) and on the left side of that the 
nerve or myelon (n.). In section 155 (Pl. III. fig. 1) we 
see the latter greatly enlarged to form the caudal ganglion 
at the base of the tail. The heart is placed just above the 
base of the tail (Pl. IV. fig. 9, h.). We see a part of it 
between the stomach and the rectum in PI. II. fig. 5, at h. 
We now come to the important region, about section 
170, where the branchial sac or pharynx communicates 
with the exterior by a pair of ventral tubular ciliated 
openings or stigmata. The series of sections represented 
by figures 3 to 6 on Pl. IIT show the structure of these 
openings and the passages leading to them. The first 
change as one approaches the stigmata is seen at a on the 
right hand side of fig. 3, (section 165) where the lateral 
edge of the branchial sac bends down ventrally while at 
the same time the ectoderm on the surface of the body 
over this region is invaginated, so that only a narrow 
bridge of tissue is left between the two cavities. The 
next stage 1s seen a couple of sections further forward at 
b in fig. 4, where a few large cells appear in the bridge of 
tissue and then become arranged, as at c, in two regular 
rows. ‘These rows of cells draw apart (see d, fig. 5), 
then additional rows appear till there are in all 4 series of 
cells with narrow slits between the rows (e. fig. 3). [On 
account of a slight obliquity in the sections or because of 
one of the stigmata being slightly further forward than 
the other, the two sides of the body show different con- 
ditions in the same section, consequently these four 
sections figured show us eight or nine different stages 
(a to 2, figs. 83 to 6)]. The middle cells of each row then 
die out as we trace the sections forwards (see /. fig. 4) 
leaving merely a set of four pyramidal richly ciliated cells 
(g. fig. 5) on each wall of the tubular passage which places 
