THE CALCIFEROUS GLANDS OF EARTHWORMS. 
467 
(b) On the Vertical Partition. — The vertical partition consists of a double cell 
layer enclosing a blood sinus (fig. 10) ; the cells on the lateral face of the partition 
are continuous with and similar to those of the lamellae ; those on the median face 
are of the nature of columnar oesophageal epithelium. The basement membrane of 
the layer on the median face is extremely stout — 3 p in thickness ; it forms the 
median boundary of the partition sinus. The basement membrane of the outwardly 
facing layer of epithelium is much thinner, and is continuous with that of the 
lamellae. The vessel (more correctly sinus) in the upper edge of the partition is 
covered by columnar cells similar to those on the median face of the partition. 
No muscular layer intrudes between the epithelial layers of the vertical partition. 
(c) The Occurrence of Rodlets on the (Esophageal Epithelium. — If a transverse 
section of any but the most anterior part of the glands be examined the columnar 
epithelium on the median face of the vertical wall will be seen to bear a layer of 
short rodlets, perpendicular to the surface of the cells ; these are stiff and straight, 
staining only lightly, of appreciable thickness, and 7 '5 p in height ; they are placed 
close together, and about six placed side by side in one plane take up a space of 10 p 
(fig. 10). The remarkable thing in their appearance is that they form a regular 
palisade, are exactly parallel to each other, and of exactly the same height. The 
free edge of the cell below the rodlets stains deeply. 
It has been said that the rodlets are not present on the vertical partition in 
the most anterior part of the gland region. They are present on the dorsal wall 
of the oesophagus, beginning rather behind the level at which they appear on the 
vertical partition ; they disappear towards the dorso-lateral region of the gut wall. 
In following a series of transverse sections from the anterior end of the glands 
backwards there will be seen, in the region in front of the rodlets, a thinner, more 
homogeneous layer on the columnar cells, in which distinct rodlets are not visible. 
This may, on the dorsal wall, simulate a cuticle, 2’5 p thick ; and it becomes trans- 
formed into the layer of rodlets as we pass backwards in the series. The same 
passage of a homogeneous cuticle- like layer into rodlets may be seen in the body of 
the glands, on the free edge of the vertical partition ; on following the epithelial 
layer over this edge the rodlets of the inwardly -looking face of the partition change 
gradually into such a layer, rather more than 2 /a thick. (Compare also the replace- 
ment of the rodlets in Helodrilus parvus by a homogeneous cuticle-like layer behind 
the region of the glands, as described later.) 
The rodlets are continued backwards for some segments behind the glands, as 
a layer 7 p high on the oesophageal epithelium. The palisade arrangement may be 
less regular, but the structures are rod-like, not hair-like. 
Rodlets are not seen on the cells of the lower portions of the lamellae of the 
glands ; but they are present on the upper halves, more or less, and on the free 
upper borders of the lamellae ; here they may have a remarkable resemblance to 
cilia, and are less regular and appear thinner than the typical rodlets. They are 
