50 A. CG. OUDEMANS. 
angular lower lacuna become gradually horizontal, forming a 
horizontal band, which separates the lateral lacune and the 
posterior lobes from the two smaller lower lacune (the one 
being now divided by vertical transverse muscular fibres into 
two). The mouth-slit becomes visible. The two lower lacune 
gradually slant downwards round the cesophagus, leaving 
behind in its folds from time to time small lacune. *The two 
upper lacunz gradually become very small, and, as the hori- 
zontal band has disappeared, they now lie on what may be 
called in the transverse section the two horns of the half-moon 
shaped intestine. The mouth-slit is now closed, and the 
cesophagus is, except the ventral side and the side towards the 
proboscidian sheath, surrounded by lacune. The upper surface 
of the median vessel in the proboscidian sheath again differs 
from the lower, rests upon the inner (longitudinal) layer of the 
sheath, is, however, not held up by small bands. It touches the 
wall of the sheath only along a very thin base (see fig. 69). 
The part in which the nephridial system is situated was not 
represented in the series I examined. The part that follows 
now lies in the intestinal region. Generative organs could be 
detected. The median vessel lies beneath the proboscidian 
sheath, and not far from the median line, on the ventral side of 
the intestine, the two ventral vessels are visible. Sometimes 
transverse vessels were very plainly to be seen, opening into 
one of the three longitudinal ones. 
From a section which was situated still further backwards, 
and very well preserved, I give a representation of one of the 
ventral vessels in fig. 66. The inner epithelial layer is dis- 
tinctly visible, also the large hyaline basal layer, thickened 
here and there by contraction, and better coloured. Following 
upon this there is a distinct layer of circular fibres, but no 
longitudinal. The stroma is radially contracted. Some cells 
in the stroma have a very distinct protoplasmic aspect. 
In a tail-portion transverse vessels were observed, even over 
the hindmost diverticulum. 
