488 Wisconsin Academy of Sciences, Arts and Letters. 
and of the fifth segments and opens into the posterior part of 
the rectum. The rectum extends forward beyond its' union 
with the ileum almost to the preceding segment; in all fresh 
specimens it was filled with air. 
A closer examination of the different parts of the alimentary 
canal, especially a study of sections, shows that there is at some 
places a sharply marked histological differentiation between 
them. The long thread-like oesophagus is of very nearly the 
same structure throughout its entire length; its diameter is at 
all parts very small and varies only at the extreme posterior 
end. The epithelial cells vary somewhat as to their relative 
length and breadth becoming very much longer at the posterior 
end, until finally just before the commencement of the mid¬ 
intestine, they are very long and narrow. (PI. XXXIV, Fig. 3.) 
Internally the oesophagus is lined with a layer of chitin; ex¬ 
ternally there are two muscular layers. The outer of these, the 
longitudinal, varies in thickness in different parts of the oesoph¬ 
agus and is follwoed by the inner circular layer. Besides 
these two muscular layers there is a small amount of connective 
tissue scattered between the muscles, separating the two muscular 
layers from each other or from the layer of epithelial cells. 
This muscular layer shows quite a change in its thickness at or 
very near the point which marks the boundary between oesoph¬ 
agus and mid-intestine. In a longitudinal section of this 
part (PL XXXIV, Fig. 3) if one follows the muscular layer a 
sudden decrease in its thickness is noticeable, the change due 
apparently to a loss of connective tissue more than of the muscles 
themselves. In all specimens examined this change occurs very 
near the spot where the internal chitinous layer stops and where 
there is also a marked change in the structure of the epithelial 
cells. This point does not coincide with that at which the 
oesophagus begins to widen but is a short distance posterior to 
this, so what one would naturally consider, from an exterior 
view, as the boundary between the oesophagus and mid-intestine 
is in fact anterior to it. The cells of the oesophagus (PL 
XXXIV, Fig. 4), excepting the variations in shape, are similar, 
containing each a spherical nucleus and very often a number of 
vacuoles in the basal part of the cell. 
The mid-intestine as seen in a dissection is readily divisible 
into two quite distinct portions, an anterior thicker part extend¬ 
ing straight through the first three abdominal segments, and a 
