THE CALCIFEROUS GLANDS OF EARTHWORMS. 
471 
cells which are relatively broader than those of the neighbouring part of the 
oesophagus proper, with more oval nuclei ; the height of the cells diminishes further 
towards the lateral wall of the pouch, where they are shortly columnar or even 
cubical, and the nuclei may be spherical ; there are no rodlets or cilia. The free 
margin of the cells of the pouches is very definite, as if each cell were bounded at 
its surface by a membrane. The blood sinus lies outside the epithelial layer of the 
pouch, and the muscular coat is very thin, hardly definitely measurable. 
In the posterior half of the pouch prominent ridges begin — better perhaps called 
lamellae. Each lamella consists of a double layer of cells, approximately cubical in 
shape ; the texture of the cells of the central portions of the lamellae appears loose 
(in the particular specimens described; but not in the H. parvus of fig. ll), as if 
the cells were disintegrating, and the cells enclose empty spaces of ragged outline. 
The cells have no rodlets or cilia ; nor have they at any place in the tunnels to 
be described. 
Some of these lamellae immediately unite at their free ends with their neighbours 
to form tunnels (fig. ll); these are at first irregular in form, but elongated in a 
direction radial to the centre of the pouch. In a short time the lamellae have all 
fused at their margins, and the series of tunnels is complete. From the first appear- 
ance of prominent lamellae to their com plete fusion to form tunnels there is a distance 
represented by only ten sections ('08 mm.). 
When the tunnels are well established, the central lumen of the pouch is much 
diminished. Each layer of a lamella has a basement membrane, and sometimes the 
blood sinus can be seen extending up between the two layers of a lamella from where 
it lies, contained externally by the muscular coat. Occasionally, flattened nuclei may 
be seen in the sinus in the lamella, lying on the basement membrane bounding the 
sinus on one or other side, or between the two cell layers of a lamella if the sinus 
does not exist or is only potential there. When first established there are about 14 
tunnels on each side, 28 in all. 
Each lamella is now the partition between neighbouring tunnels ; it contains in 
its axis, potentially at any rate, an extension of the gut sinus, which lies primarily 
internal to the muscular layer of the alimentary tube ; and by the fusion of the 
central ends of the lamellae a layer of epithelium lining the lumen of the oesophagus 
has been established, which is no longer directly continuous with the epithelium of 
the lamellae. 
Meanwhile the tunnels have been encroaching on the dorsal and ventral portions 
of the oesophageal wall, more rapidly on the dorsal than on the ventral wall ; or, 
in other words, what was called above the oesophagus proper — the portion of the 
alimentary tube between the lateral pouches — has almost disappeared. Where the 
oesophagus passes through septum 10/11 tunnels surround the whole except in the 
mid-ventral line ; they are now restricted in height, their section being shortly oval. 
The Alimentary Canal in segment xi . — In the next segment after the oesophageal 
TRANS. ROY. SOC. EDIN., YOL. LII, PART II (NO. 17). 72 
