126 Invertebrate Zoology. 



The external opening of the silk glands has already been 

 noted. 



Make a drawing of the alimentary tract. 



The Excretory System. A series of irregular tubes 

 will be observed lying upon the posterior half of the mesen- 

 teron, each pursuing a somewhat irregular course and pre- 

 senting, when seen under the lens, a beaded appearance. 

 These organs are the Malpighian tubes, six in number. Each, 

 after pursuing a tortuous course, empties into the intestine. 



Make a drawing of the excretory system. 



The Respiratory System. The spiracular openings 

 have already been noted. Each leads into a series of branch- 

 ing tubes, the tracheae, which subdivide into very minute 

 subdivisions, carrying the air to the most remote tissues. 

 The tracheae are lined with chitin and are strengthened by a 

 spirally coiled chitinous ridge. 



third thoracic or first abdominal segment along the lateral surface of 

 the anterior portion of the digestive tract, in its course being partly 

 covered by the lobes of the fat-body. 



The salivary duct passes down the posterior surface of the bundle 

 of muscles that works the jaws. 



The silk glands are much larger than the salivary glands and occupy 

 a very different position. Their ducts lie immediately below the oesopha- 

 gus and the anterior portion of the stomach. The greater portion of 

 each gland appears as a coiled cord, more or less surrounded by the fat- 

 body and lying on a level with the spiracular openings, in the middle 

 and posterior portions of the body-cavity. 



The salivary and silk glands should not be confused with the much 

 more delicate Malpighian tubes, which are arranged in more nearly 

 parallel lines along the outer surface of the stomach. 



