264 CHORDATE ANATOMY 



similar in Protozoa and Metazoa. A true vascular system, however, is 

 multicellular and therefore limited to the Metazoa. 



The simpler metazoans, 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 alimentary 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 Hning 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 in mass of tissue, as well as by the separation of the body- wall 

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

 vessels are represented in hving invertebrates. 



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

 such as occurs in most invertebrates and a closed system Hke 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 

 sort of lymph, without blood corpuscles and without hemoglobin. It may 

 be assumed that the walls of these vessels are formed directly from the 

 surrounding mesenchyma and that the vessels therefore are evolved from 

 lacunar spaces. In the nematodes, a pseudocoelom provides an adequate 

 mechanism of circulation in these animals which have no thick masses of 

 tissue to nourish. 



Most of the invertebrate phyla above the nematodes have composite 

 circulatory systems, partly lacunar and partly closed. Before Malpighi 

 (1661) discovered the capillary circulation in vertebrates and thereby 

 demonstrated in them a closed circulation, it was assumed that the verte- 

 brate circulatory system was likewise partly lacunar and partly closed. 

 That the lacunar system of invertebrates is comparable with the lymphatic 



