THE ABDOMEN. 



119 



other by valves, which allow the fluid to go forward 

 to the head, but not back towards the other ex- 

 tremity. Passing by a simple elastic tube through the 

 thorax, the blood, if we may so call it, is propelled to 

 the anterior segment of the body. Its subsequent 

 course is not very clear ; for, while some anatomists 

 speak of a small vessel leading back to the hinder part 

 of the body, others consider that the sanguineous, or 



FIG. 35. a, TRACHEAE ; b, ELASTIC SPIRAL OF TRACHEAE. 



nutritive, liquid finds its way from the cephalic parts 

 to other vital organs, and after bathing them, returns 

 to the dorsal vessel by a second set of valves per- 

 mitting its ingress only. 



Turning next to the nutritive organs, we have 

 already spoken at sufficient length of the mouth and 

 its appendages, and have mentioned that the nectar 



