DEVELOPMENT OF THE VASCULAR SYSTEM. 373 



system. The Ampliioxus, as has been stated, has no real 

 heart ; but the blood is circulated in its vascular system by 

 the main vascular stems themselves, which contract and 

 pulsate along their whole length. (Cf. Fig. 151, vol. i. p. 420.) 

 A dorsal vessel (aorta), situated over the intestine, absorbs 

 the arterial blood from the gills and propels it through the 

 body. The venous blood, in its return, collects in a ventral 

 vessel (intestinal vein), situated under the intestine, and 

 thus returns to the gills. Numerous vascular gill-arches, 

 which accomplish respiration, and absorb oxygen from the 

 water and emit carbonic acid, unite the ventral vessel 

 with the dorsal vessel before. As, in Ascidia, that section 

 of the ventral vessel which also forms the heart in Skulled 

 Animals (Craniota), is already fully developed into a simple 

 heart-pouch, we must regard the absence of the latter in the 

 Amphioxus as the result of retrogression, as a reversion, in 

 these Acrania, to the older form of vascular system, as it 

 exists in Scolecida and many other Worms. We may 

 assume that those Acrania which actually formed part of 

 our ancestral line did not share this relapse, but rather 

 inherited the one-chambered heart from the Chordonia and 

 transmitted it directly to the older Skulled Animals 

 (Craniota). 



The Comparative Anatomy of Skulled Animals clearly 

 exhibits the further phylogenetic development of the blood- 

 vessel system In the lowest stage of this group, in the 

 Cyclostoma (p 102), we first meet with a real lymph-vessel 

 system, side by side with the blood-vessel system, a system 

 of canals which collect the colourless fluid flowing from the 

 tissues, and conduct it to the blood-current. Those lymph- 

 vesjsels which absorb the milky, nutritive fluid, obtained 



