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A. M. REESE 
with the naked eye than is shown in the figure. The pigmentation 
is chiefly due to great numbers of chromatophores having the 
shape of typical spider or mossy cells. 
The actual slit, as seen in fig. 5, s, is very narrow, though it 
may vary somewhat, in different regions. Its outline is very 
irregular, though this also varies in different regions. The 
cause of these small, irregular projections will be seen a little 
later. 
The canals of the second type (fig. 6) are five to ten times larger 
than those of type 1, although, superficially, they do not appear 
so because of the light border, noted above, in connection with 
the latter type. As seen with the naked eye or under a lens the 
canals of type 2 have somewhat the appearance of fig. 6. Around 
the inner margins of the wide spaces, a short distance below the 
level of the surface (fig. 6, w), is sl sort of shelf that partly closes 
the opening. In some canals this shelf extends along the narrow 
part of the canal, between the widenings, as is shown in fig. 6. 
The meaning of this shelf will be explained below. The wide spaces 
are not alwa^^s so symmetrical as those shown in the figure; in 
some places the widening is all on one side of the canal; and in 
other places, the widenings of the two sides do not lie exactly 
opposite each other. 
The structure of the canal is best made out by the study of 
transverse sections, but some features can be seen in longitudinal 
sections, and in pieces of the entire canal that have been cleared 
for study under the low powers of the microscope. Serial sections 
were made of eight or ten different regions of the canal system. 
Both the paraffin and the celloidin methods were used, the latter 
giving the better results. 
Figs. 7 to 14a area series of semi diagrammatic camera-drawings 
of cross sections of different regions of the system (see Description 
of Figures). Figs. 14, 14a were drawn under a magnification of 
fourteen diameters, while all the rest were magnified forty-one 
diameters. It will be noticed that all the sections of canals of type 
1 (figs. 7-13) are of about the same size and character (the differ- 
ence in shape is probably due mainly to fixation), except that the 
-section from the tip of the tail (fig. 7) is much smaller than the 
