62 



ZOOLOGY. 



suctorial insects, and long in those that feed on vegetables. 



The intestine ends in a 

 cloaca, which also receives 

 the products of the genital 

 organs. 



The oesophagus is usu- 

 ally furnished with tubular 

 organs, which are believed 

 to be salivary glands. 

 There are certain other 

 tubes connected with the 

 intestine which are thought 

 to be the representa- 

 tives of the liver and 

 kidneys. 



72. Circulation. The 

 heart of insects is in the 

 form of a long tube. It 

 is placed along the back, 

 and, on this account, is 

 called the " dorsal vessel." 

 It consists of eight or nine 

 sacs opening into one an- 

 other, and connected by ; 

 valves which allow the 

 blood to pass towards the 

 head, but not in the oppo- 

 site direction. The blood 

 received into the heart is 

 driven for wards by the con- 

 traction of these sacs, until 

 it escapes near the head. As there are neither veins 

 nor arteries, the blood is conveyed back to the heart 

 through the interstices of the tissues. 



73. Respiration is effected by a series of branched 

 tubes called tracheSB (Gr. trackeia, the windpipe), which 

 convey air through every part of the body. They com- 

 municate with the atmosphere by a number of slits, 



Fro. 24. RESPIRATORY ORGANS OF AN 

 INSECT. 



Distribution of the tracheae in the 

 thorax and abdomen of the Nepas. Stig- 

 mata and air sacs. 



