CUTICULAR TISSUE. 195 
the strie are the optical indications of parallel undulating 
canals which traverse the successive strata of the cuticle, 
and are ordinarily occupied by a fluid. When this dries 
up, the surrounding air enters, and more or less com- 
pletely fills the tubes. And that this is really the case 
may be proved by making very thin sections parallel with 
the face of the exoskeleton, for these exhibit innumerable 
minute perforations, set at regular distances from one 
another, which correspond with the intervals between the 
strize in the vertical section ; and sometimes the contours 
of the arew which separate the apertures are so well 
defined as to suggest a pavement of minute angular 
blocks, the corners of which do not quite meet. 
When a portion of the hard exoskeleton is decalcified, 
a chitinous substance remains, which presents the same 
structure as that just described, except that the epios- 
tracum is more distinct; while the ectostracum appears 
made up of very thin lamin, and the tubes are repre- 
sented by delicate striw, which appear coarser in the 
region of the dark zones. As in the naturally soft parts 
of the exoskeleton, the decalcified cuticle may be split 
into flakes, and the pores are then seen to be disposed 
in distinct ares circumscribed by clear polygonal borders. 
These perforated are appear to correspond with indi- 
vidual cells of the ectoderm, and the canals thus answer 
to the so-called ‘“‘ pore-canals,” which are common in 
cuticular structures and in the walls of many cells 
which bound free surfaces. 
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