408 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 





Blood System. — Insects are very highly organized 

 animals, and yet their blood system is very simple and 

 incomplete, far more so than in mollusca or even echino- 

 derms. 



Along the dorsal aspect of the body, immediately be- 

 neath the chitinous shell, there is a long, valvulated, 

 pulsating vessel — dorsal vessel (Fig. 287). This may be 

 called the heart. The valvules are so arranged as to 

 direct the course of the blood coi\X\x\\i2.\\y forward. This 

 dorsal vessel divides into several arterial branches at its 

 anterior end, and receives several venous branches at 

 its posterior end. The anterior branches discharge into 

 the tissues forward, while the posterior branches suck 

 in from the tissues behind. This is apparently all that 

 there is of true vessels. In verte- 

 brates the blood system is a closed 

 system of pipes. In the higher in- 

 vertebrates there are indeed a 

 number of reservoirs scattered 

 about the body ; but still the blood 

 system is essentially a vascular sys- 

 tem. But in insects there are no 

 vessels at all except the large ves- 

 sels of the heart, but the tissues 

 are everywhere full of minute in- 

 tratissue spaces (lacuna) connected 

 with one another in all directions. 

 Now, by the continuous discharge 

 of blood in front and the sucking 

 up of blood behind, it is evident 

 that the blood must work round 

 among the tissues without definite channels, but in a 

 general way backward, to be taken in again into the 

 heart and forced forward. This, therefore, is called lacu- 

 nary circulation. There are also probably valvular open- 





Fig. 287. — Diagram show- 

 ing the heart and general 

 course of the circulation 

 in insects. 



