CIRCULATION IN INSECTS. 



101 



or close to the rectum. Lastly we find, towards the extremity 

 of the intestinal canal, other secreting organs, that serve to 

 elaborate particular liquids (such as the poison of the Bee's 

 sting), which several Insects can throw out from the extre- 

 mity of their abdomen, when they are irritated. 



618. It would appear to be by simple absorption, that the 

 chyle traverses the walls of the digestive tube and mixes with 

 the Blood. This liquid is watery and colourless; it is not 

 everywhere inclosed in vessels, but spreads itself out into the 

 interstices, which exist between distinct organs, or which pre- 

 sent themselves in the substance of their tissue. Insects are 

 destitute of a regular circulation. We can clearly see very 

 rapid currents in certain parts of the body ; but the nourishing 

 liquid does not perform a circle in such a way as to return 

 constantly to the point of its departure. There really exist 

 amongst these animals only vestiges of a circulating apparatus. 



Fro. 325. CIRCULATION IN INSECTS. 



We see near the dorsal surface of the body a longitudinal tube 

 (a, Fig. 325), which performs alternate movements of contrac- 

 tion and dilatation, analogous to those of the heart among the 

 higher animals. The nourishing liquid penetrates into it by 

 lateral openings, furnished with valves to prevent its reflux ; and 



