THE VASCULAR SYSTEM. 



371 



ascular loops, within the wall of this gill-body, which 

 passed from the ventral vessel to the dorsal vessel, became 

 modified into respiratory gill-vessels. Even at the present 

 day, the organization of the remarkable Acorn-worm 

 (Balanoglossus) exhibits a similar condition of gill-circula- 

 tion (Fig. 186, p. 86). 



A further important advance is exhibited, 

 among extant Worms, in the Ascidia, which 

 must be regarded as the nearest blood-rela- 

 tions to our primitive Chordonia ancestors. 

 In these we find, for the first time, a real 

 heart, that is, a central organ of tlie circula- 

 tion of the blood, by the pulsating contractions 

 of the muscular wall of which the blood is 

 driven forward in the vascular tubes. The 

 heart appears here in the simplest form, as 

 a spindle-shaped pouch which passes at both 

 ends into a main vessel (Fig. 188, c. p. 90; 

 Plate XI. Fig. 14, hz). The original position 



Fro. 298. Blood-vessel system of a Ringed Worm 

 (Saennris) ; front section : d, dorsal vessel ; v, ventral 

 vessel ; c, transverse connection between the two (en- 

 larged like a heart). The arrows indicate the direction of 

 the blood current. (After Gegenbaur.) 



of the heart on the ventral side, behind the gill-body of the 

 Ascidian, plainly shows that it originated in a local dilation 

 of a section of the ventral vessel. The alternating direc- 

 tion of the movements of the blood, which has already been 

 mentioned, is remarkable ; the heart expels the blood alter- 

 nately through the anterior and through the posterior end. 

 This is very suggestive, because in most Worms the blood 



