THE VASCULAR SYSTEM 



349 



The simpler metazoa, such as the sponges and coelenterates, are devoid 

 of a vascular system. In an animal such as hydra in which the body-wall 

 forms the aUmentary canal, a vascular system is unnecessary. With only 

 two layers of cells in the body-wall, the diffusion of food stuffs from the 

 digestive cavity into the cells may take place by osmosis. The excretion 

 of wastes is likewise direct and requires no special system of transportation. 



A circulatory system is necessary and is present in all animals in which 

 the body-wall is separated from the lining of the aUmentary canal either 

 by a mass of mesoglea or by a body-cavity. In other words, the emer- 

 gence of a circulatory system in animals is conditioned by increase in size of 

 body and of mass of tissue, as well as by the separation of the body-wall 



Fig. 293. — Diagram of embryonic circulation in a large-yolked vertebrate, aa, 

 aortic arches; al, allantois; an, anus; ca, cv, caudal artery and vein; da, dorsal aorta; 

 dc, Cuvierian duct; h, heart; ha, hypogastric (allantoic) artery; j, jugular vein; I, liver; 

 oma, om, omphalomesenteric artery and vein; pc, postcardinal vein; si, subintestinal 

 vein; st, sinus terminalis; va, ventral aorta; y, yolk; ys, yolk stalk. (From Kingsley's 

 "Comparative Anatomy of Vertebrates.") 



from the alimentary canal by a coelom. Stages in the evolution of blood 

 vessels are represented in living invertebrates. 



Metazoa have two kinds of vascular systems, an open lacunar system 

 such as occurs in most invertebrates and a closed system like that of 

 vertebrates. The facts support the assumption that the lacunar system 

 is the more primitive. A lacunar system is well represented in flatworms, 

 in which a fluid plasma fills the spaces between loose mesenchymatous 

 cells. No heart is present and no true circulation occurs. The contraction 

 of the muscles of the body wall and the movements of the worm bring 

 about more or less irregular currents in the plasma. In many flatworms, 

 numerous diverticula of the intestine bring digested food near most parts 

 of the body so that a vascular system is unnecessary. The beginnings of 

 blood vessels, however, make their appearance in nemerteans which are 

 sometimes classified with flatworms. Nemerteans have in addition to 

 lacunar spaces in the mesenchyma three longitudinal blood vessels, two 

 lateral and one dorsal. Interconnexions between these vessels occur at 

 the anterior end of the worm. The fluid contained in these vessels is a 



