208 LECTURE V. 



though not sufficiently explanatory, must, 

 I think, principally have been suggested 

 by what is observable in animals of this 

 class. I am, however, aware that the mo- 

 tion of the blood in the vessels of the 

 chicken and its appendages before they are 

 conjoined, and a circulatory motion conse- 

 quently established, might have suggested 

 the same expression. In worms, we see 

 the dorsal vessel pretty suddenly filling, 

 till it is distended with blood ; it then con- 

 tracts, urging on its contents, till it is 

 empty, when, after a time, it is again reple- 

 nished. Thus wave follows wave, but the 

 cause of this interrupted, or seemingly un- 

 dulatory motion, has not perhaps been fully 

 explained. Analogy would induce us to 

 consider the systole and diastole of the 

 dorsal vessel, as similar to what is observed 

 in the heart of the higher classes of ani- 

 mals, and that the tardiness of the circu- 

 lation is the cause of this apparently un- 

 dulatory motion. 



The spiracula or tracheae in the sides of 

 worms terminate in vesicles, which are con- 



