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-wonn 

 (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 the 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 (w ^^tll \ 

 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. 00; 

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



Fig. 298. — Blood-vessel system of a Ringed Worm 

 (Saer.uris) ; front section : d, dorsal vessel ; r, 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 



