CHAEACTEES OF TEETEBEATB ANIMALS. 283 



is of a very primitive type compared to the blood-system of the 

 Vertebrata. In all Craniota there is a contractile heart, which 

 never consists of less than two distinct chambers (fig. 166), with 

 valvular openings ; more generally the heart is composed of 

 four distinct divisions (fig. 162). In the lowest vertebrates 

 this heart only pumps the blood to the respiratory organs to be 

 purified, but in the higher forms there are two distinct vascular 

 systems — one by which the blood is pumped to the lungs and 

 then back to the heart, called the " pulmonary circulation " ; 

 the other by which the blood that has been to the respiratory 

 organs is sent to the various parts of the body, the so-called 

 "systemic circulation." Again, in this great sub -kingdom 

 there is always present a modification of the venous system 

 known as the hepato-jjortal system, by which some of the blood 

 sent to the intestinal portion of the alimentary canal is sent by 

 a tube called the vena porta back to the liver, and not direct to 

 the heart. 



In connection with the venous system is also found in all 

 Craniota a set of vessels along the walls of the gut, which take 

 up the products of the intestinal digestion and pour them into 

 the great vein, or vena cava. This system of vessels is known 

 as the lacteal system. The substance contained in this system 

 is chyle, and the main duct opening into the vena cava is 

 designated the thoracic duct (fig. 162, Td). There are also 

 certain embryonic characters common to the vertebrates. In all 

 embryos we find two structures present — namely, the " primitive 

 groove " and the " visceral arches.'' The primitive groove 

 makes its appearance in the ovum early in development ; it is 

 found as a long depression in the germinal area of the egg, 

 with raised lip-like sides ; and at the bottom of this canal the 

 notochord is seen. These early stages of development will be 

 treated more fuUy in the Embryology of the Fowl, where we 

 shall observe that at no time does the nervous system surround 

 the gut anteriorly as in the lower animals, and that it com- 

 mences as an open tube. 



