H. ORGANS OF CIRCULATION. 



(VASCULAR SYSTEM.) 



IN Amphioxus, the vessels are of a simpler type than in the 

 Craniata, and to a certain extent retain characters only seen m the 

 embryos of the latter. There is no heart, and the circulation of 

 the blood is effected by the peristaltic contraction from behind 

 forwards of the ventral blood-vessel (ventral aorta). 



In the Craniata the vascular system, which arises from the 

 mesoderm, consists of a hollow central muscular organ, the heart, 

 which is connected with a series of closed tubes, the blood-vessels, 

 containing a coloured fluid, the blood : there is also another system 

 of vessels containing a colourless fluid, the lymph, which, however, 

 besides permeating all the tissues, is present in various spaces or 

 sinuses in the body as well as in the lymph-vessels. The 

 lymphatic system is therefore not completely closed, the vessels 

 communicating with the sinuses on the one hand, and with the 

 blood-vessels on the other. The lymph-vessels coining from the 

 intestine are known as lacteal s. 



The blood, which serves to carry the absorbed food and oxygen 

 to, and the waste products from, all parts of the body, is kept in 

 constant circulation through the vessels by the rhythmic contraction 

 of the heart, which acts both as a force-pump and a suction-pump. 



All the blood-vessels which bring back the blood to the heart 

 are known as veins, while those which carry it from the heart 

 are called arteries : the latter usually contain oxygenated blood 

 of a bright red colour, the former impure, darker blood, rich in 

 carbon dioxide and other products of destructive metabolism ; but 

 this is by no means always the case. Many of the veins, and also 

 of the lymph-vessels, are provided with valves, which are adapted 

 to prevent the reflux of the blood : they usually have the form of 

 semilunar folds of the internal coat, and are so arranged that two 

 are placed opposite to one another. The arteries (and also certain 

 of the veins) divide up into smaller and smaller branches, eventually 

 giving rise to microscopic tubes called capillaries, the walls ot 

 which consist of a single layer of epithelial cells, surrounded by 

 contractile structures analogous to the smooth muscle-fibres of the 

 larger vessels and consisting of branched muscle-cells which are 



