CIRCULATION IN THE ANNELIDA. 233 



SO as to constitute short circles subordinate to the 

 general or systemic circulation, which is carried on 

 by the dorsal and abdominal trunks. In this plan 

 the dorsal vessel must be regarded as arterial and 

 aortic, that is, supplying the general system : while 

 the abdominal vessel has assigned to it the office of 

 a great venous trunk, collecting the blood which 

 has circulated in the capillaries of every part, and 

 returning it partly to the dorsal vessel, and partly 

 by extensive transverse branches to the two lateral 

 trunks, which convey the portion they receive to 

 the subcutaneous respiratory organs, in the manner 

 already described. 



These movements of the blood, in all the pre- 

 ceding instances, appear to be effected entirely by 

 the contractions of the vessels in which it flows : 

 unassisted by the additional force which, in other 

 cases, it derives from the muscular power of di- 

 lated portions of those canals. In proportion as 

 enlarged cavities are provided, these then become 

 more and more the principal agents of propulsion. 

 Such, in the Eu?iic€, is the function of certain 

 bulbous expansions of the transverse branches of 

 the abdominal vessel. The Terebella presents a 

 still more complicated mechanism, composed of two 

 distinct sets of propelling organs; the one belonging 

 to the dorsal system, and driving the blood to the 



of arteries or veins, under which they have described particular sets 

 of vessels; such designations being calculated to mislead, in conse- 

 quence of their not being descriptive of the real office of these vessels, 

 and of the true place they occupy in the system of the circulation. 

 But there is, moreover, reason to believe that, in many instances, the 

 course of the blood in these animals is liable to variation, the currents 

 being occasionally reversed from their usual directions, producing 

 an undulatory instead of a constant motion. 



