4o8 



HISTOLOGY 



position by moving down out of this layer as do the acid cells from the 

 epithelium of the gastric glands of the stomach of mammals. 



The Myriapoda have odorous glands which produce hydrocyanic 

 acid gas. Julus is the best example and the gland is exceedingly simple. 

 It is again an integumental invagination, a simple, saccular form, the 

 color of whose tissues gives it a dark, purple shade. Its duct possesses 

 a most interesting closing device, as we must call it, because its elasticity 

 causes it to remain closed until the muscle fiber opens it (Fig. 369). 

 The wall of the duct is made of an inner chitinous layer and an outer 

 cellular layer, the hypodermis. Near its exit, one side of this tube is 

 evaginated into the lumen of the tube, and the lower end of this process 

 is forced down into the external opening. This is its natural position, 



FIG. 369. Three sketches to illustrate the apparatus by which the discharge of prussic acid 

 from its repugnatorial glands is controlled by Julus. A, general side view with inside out- 

 lines indicated by dotted lines; stopper in place. B, slightly oblique section to show outer 

 opening of duct and the stopper withdrawn. C, transverse section to show relations of 

 muscle and stopper. Lettering for all figures : m., mouth or outer opening of tube; s., stop- 

 per; mus., muscle by whose contraction the stopper is withdrawn; cu., outer surface of cu- 

 ticle, seen from the surface in A and from the side, in section, in B and C. (After Rossi.) 



and so the tube is closed and stoppered when at rest. A muscle fiber 

 from some point of attachment on the ring segment enters the hollow 

 lumen of this evagination, and when it pulls, the stopper is withdrawn, 

 and the tube acquires a horseshoe-shaped lumen which permits the 

 secretion to escape. 



Beneath the cuticular lining of the sac lies the secreting epithelium. 

 Unlike that of the two insect examples we have been studying, this is 

 a simple, cuboidal epithelium. It secretes the poisonous and offensive 

 hydrocyanic acid which one can smell when handling the creature. 



Many insects give off an odor that is not only agreeable to other in- 

 sects but to man as well. They are distinctly alluring glands. These 

 glands are found in still more peculiar places in the anatomy of the crea- 

 tures than even the offensive glands were. A favorite position in the 



