NOTES ON THE STRUCTURE OF OIKOPLEURA. 49 
side, leaving only its middle third uncovered. In the 
dorsal part of the branchial sac the two ciliated pads 
which further back lay one at each side of the narrow 
epipharyngeal groove are now seen to have moved further 
ventrally, and they continue as traced forwards to run 
obliquely down the lateral walls of the branchial sac (see 
figs. 7and 8), being now in fact the peripharyngeal ciliated 
bands of ordinary ascidians, till finally they meet in the 
ventral median line in the region of the endostyle. In the 
middle of the lateral surface of the body (fig. 8) the ecto- 
derm is more than one layer deep. ‘There are large 
columnar cells,* in each of which several nuclei are present 
while at the bases of these there are smaller triangular 
cells with their apices running in between the adjacent 
columnar ones (PI. III. fig. 10). 
The tail in the last few sections while remaining of about 
the same width from side to side has become a good deal 
thicker dorso-ventrally, and the dorsal and ventral surfaces 
in the middle third have become covered by a thicker 
layer of ectoderm, while inside that and co-terminous with 
it is a well marked layer of muscle fibres (see Pl. III. figs. 
8 and 9). In the centre of the tail and occupying all 
the extent between the two muscle layers is the notochord 
with its usual undulating outline, and on its left side the 
delicate nerve cord (see Pl. III. fig 7). The tail retains 
this structure through all the sections forwards to and in 
front of the oral end of the body proper. 
The next section figured, No. 216 (PI. IIL. fig. 8), shows 
the body becoming rapidly smaller, while the ectoderm 
cells on the lateral walls have become enormous, and 
*It is probably these cells which Sanders (Mon. Micros. Jour., Ap. 1874, 
p. 141) mistook for stigmata in his Oikopleura from Torquay. Possibly the 
supposed gill-slits in Moss’s remarkable appendicularian (Linn. Trans. XXVII_ 
p- 299.) may have been either large ectoderm cells or large glandular cells of 
the endostyle. 
* 
