THE VASCULAR SYSTEM IN TRIMITIVE VERTEBRATES. 265 



the system of blood-vessels and the system of lymj)hatic 

 vessels. The cavities of the former contain the red blood ; 

 those of the latter, the colourless lymph. To the lymphatic 

 system belongs the coelom (the so-called pleuro-peritoneal 

 cavity) ; and also numerous lymphatic ducts which extend 

 through all the organs, absorbing the juices which have 

 been consumed from the tissues, and conveying them into 

 the venous blood. Finally, the chyle- vessels, which absorb 

 the white chyle or milky nutritive juice prepared by the 

 intestines, carry it into the blood. 



The blood-vessel system of Vertebrates is developed in 

 various ways, but seems originally to have existed, in the 

 Primitive Vertebrate, in the simple form in which it now 

 permanently exists in the Ringed- worms (Annelida) — for 

 example, the common Earth-worm — and in the Amphioxus. 

 Two large unequal blood-channels, which are originally 

 situated in the fibrous wall of the intestine, and which run 

 along the intestinal canal in the central plane of the body 

 (one underneath the intestinal canal, and the other above), 

 must especially be regarded as essentially and originally the 

 most important part of the blood-vessel system. These two 

 principal channels give rise to many branches which traverse 

 all parts of the body, and pass into each other in curves at 

 the anterior and posterior ends of the body ; we will call 

 them the primitive artery and primitive vein. The former 

 represents the dorsal vessels, the latter the ventral vessels 

 of the Worms. The primitive artery or primordial aorta 

 (Fig. 59, -a) lies on the top of the intestine, along the central 

 line of the dorsal side, and conveys oxygenated or arterial 

 blood from the gills into the body. The primitive or 

 primordial principal vein (Fig. 60, v) lies below the intes- 



