198 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



branches ; and in the most complicated form the branches 

 are ornamented with hiteral branchlets. For a certain 

 distance from the base of the seta, its sm-face 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 alwaj^s 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 setae, however, the parietes, near the 

 base of the seta, are thickened in such a manner as 

 almost, or completely, to obliterate the central cavity. 

 How^ever thick the cuticle may be at the point from 

 which the setae 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. 



