THE COMMON COCKROACH 



digestive system, we have yet to notice some important 



accessory organs, the salivary glands, which in the cock- 



roach are enormously developed. On opening the body, 



they may be seen lying along just outside the walls of 



the crop in its anterior part. Each 



consists of a pair of white glands 



(Fig. 41), and a very thin -walled 



elongated bag or receptacle, quite 



distinct from these. Each gland, 



though apparently compact, really 



consists of a flattened tree-like 



structure, the foliage portions of 



which contain the cells which se- 



crete the saliva, while the branches 



are minute tubes which convey it, 



when secreted, from the gland. A 



narrow tube, or " duct," formed by 



the union of the above branches, 



receives the salivary fluid from FIG. 4 i. Salivary Glands of 



each gland; but the two ducts of 



each side soon unite into a single 



one, and the two thus formed 



again coalesce and form one main central tube lying 



under the oesophagus. 



Similarly the ducts of the receptacles unite in a 

 single central canal, which receives a little beyond this 

 junction the common duct of the glands, and so finally 

 only one tube remains as the representative of the 

 original six, and this opens into the mouth behind the 

 tongue. 



Like other terrestrial insects, a cockroach breathes by 

 taking in air, not through its mouth, but at certain 

 openings in the sides, called spiracles, or stigmata. 

 There are ten of these on each side, eight pairs being 



tacles- 



