NOTES ON THE STRUCTURE OF OIKOPLEURA. 45 
and the intestine is now seen to be completely separated 
off from the pyloric sac, while the cecal extremity of a 
new tube—a diverticulum running posteriorly and to the 
left from the rectum is seen in the middle line between 
the stomach and the intestine. In the next section 
figured (Pl. II. fig. 4), which is just ten further forward, 
this diverticulum is seen as a large distinct tube lying 
near the centre of the body and between stomach and 
intestine. It can be traced alongside the latter (see Pl. 
II. fig. 5) for about 20 sections when (PI. II. fig. 6) the two 
tubes communicate, and finally after another 20 sections 
the two have completely coalesced to form a single medio- 
ventral tube, the rectum. PI. II. fig. 6 (section 134) shows 
the two tubes communicating by a narrow passage pro- 
vided with long cilia; Pl. II. fig. 7 (section 151) shows the 
tubes almost completely joined there being only a small 
chink left to represent the intestine, the greater part of 
the lumen being derived from the diverticulum ; finally 
Pl. III. fig. 1 (section 155) shows the median rectum, still 
a little unsymmetrical in shape. It results from this 
arrangement seen in the last 50 sections that there must 
be only a very narrow slit-like opening from the intestine 
into the point of junction of the rectum and its caecum. 
These sections (103 to 155) show also the gradual but 
marked growth of the pharynx in size and differentiation 
as it is traced forwards. Figures 4 and 5 on PI. II. and 
fig. 1 on Pl. IIL. show the triangular shape, the broad 
ventral base, the thin lateral walls, and the formation of 
ciliated tracts dorsally and ventrally, the dorsal being a 
pair of prominent pads which bound the epipharyngeal 
groove (see Pl. II. fig. 5, ep.p.), while the ventral is a flat 
hypopharyngeal plate (hy.p.) which can be traced forwards 
to near the endostyle. Section 113 (Pl. II. fig 4) shows 
the anterior bifurcation of the testis (sp.) which dies out 
