198 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
branches ; and in the most complicated form the branches 
are ornamented with lateral branchlets. Fer a certain 
distance from the base of the seta, its surface is usually 
smooth, even when the rest of its extent is ornamented 
with scales or branches. Moreover, the basal part of the 
seta is marked off from its apical moiety by a sort of 
joint which is indicated by a slight constriction, or by a 
peculiarity in the structure of the cuticula at this point. 
A seta almost always takes its origin from the bottom of 
a depression or pit of the layer of cuticle, from which it is 
developed, and at its junction with the latter it is generally 
thin and flexible, so that the seta moves easily in its 
socket. Each seta contains a cavity, the boundaries of 
which generally follow the outer contours of the seta. In 
a good many of the sete, however, the parietes, near the 
base of the seta, are thickened in such a manner as 
almost, or completely, to obliterate the central cavity. 
However thick the cuticle may be at the point from 
which the sete take their origin, it is always traversed 
by a funnel-shaped canal (fig. 56, B, d), which usually 
expands beneath the base of the seta. Through this 
canal the subjacent ectoderm extends up to the base of 
the seta, and can even be traced for some distance into 
its interior.. 
It has already been mentioned that the apodemata and 
the tendons of the muscles are infoldings of the cuticle, 
embraced and secreted by corresponding involutions of 
the ectoderm. 
