CIKCULATION IN THE LALNCELET. 421 



wall of the anterior intestine, and from -w-^hich the blood- 

 vessels proceed, there is in the Amphioxus no spi^cial central- 

 ized heart, propelling the blood by its pulsations. . Instead, 

 fche movement of the blood in the Amphioxus, as in £Kr 

 Ringed Worms (Annelida), is effected by the thin tubular 

 blood-vessels themselves, which perform the functions of the 

 heart, contracting and pulsating through their entire length, 

 and thus driving the colourless blood through the whole 

 body. This circulation is so simple and yet so remarkable, 

 that we will briefly consider it. Let us begin in front at 

 the lower side of the gill-body. In the central line of this 

 lies a large main vessel, which corresponds to the heart of 

 other Vertebrates and to the main gill-artery proceeding 

 from its heart, and which propels the blood into the gills 

 (Fig. 151, 1). The anterior portion of this is swollen like 

 a heart and is extended (immediately in front of the first 

 gill-opening). Numerous little arching vessels rise on each 

 side from this gill-artery, form little heart-like swellings 

 (bulbs, m) at their point of departure, traverse the gill- 

 arches, between the gill-openings, round the anterior intes- 

 tine, and unite as gill-veins above the gill-body in a great 

 main vessel, which passes below the notochord. This vessel 

 is the primitive aorta (Plate X. Fig. 13, ^ ; Plate XI. 

 Fig. 15, t). The aorta passes between the intestine and the 

 notochord precisely as in all the higher Vertebrates. The 

 branch-vessels which this aorta sends to all parts of the 

 entire body, again collect into a large venous vessel, which 

 passes to the lower side of the intestine, and which may 

 here be called the intestinal vein (Fig. 151, o ; Plate X. 

 Fig. 15, -y ; Plate XL Fig. 13, v). It passes on further over the 

 pouch-like liver, there forms 8j kind of cystic vein, weaving 



