1938. So long as intestine and skin were one in kind, this vascular process was in every situation. With their separation therefore the vascular structure has of necessity originated between two opposed situations. There is no point in the skin and none in the intestine, where there might not be an air-and a water-vessel, a respiratory and absorbent duct. 1939. There are therefore _numerous_ vessels, and consequently a _Vascular system_. 1940. Air-and water-vessel must abut against each other; because they are polar, because the one leads to this place and the other to that. 1941. The system of water-and air-ducts can form no closed vascular system; for they only grow towards each other, as did formerly intestine and skin. 1942. They would not have originated if both cysts had not separated from each other. These vessels are not therefore to be met with in animals that have no intestine. The transition of the water-into the air-ducts takes place in the higher animals through the union of the thoracic duct with the subclavian vein, which conveys the blood directly to the lungs. 1943. The vascular system is properly the primo-cellular tissue, which occupies the middle, and at whose extremities the two cysts remain approximated, in order that they may continue to live. 1944. The vascular is the original system, since its two extremities already carry in themselves the air-and mucus-process, so that branchia and intestine are only to be viewed as peculiarly perfected conditions of these extremities. I would even have developed the vascular system first of all, had not another course been required to render the subject intelligible. 1945. The first animal cyst is a vessel with two kinds of extremities, whereof, like the plant, one draws its supply from the water, the other from the air; but in the animal this is effected by its own motion. 1946. The branchia is nothing but a vascular tissue in the air, the intestine none other than a vascular tissue in the water. Thus, do I wish that these organs should be understood, and not as mere cystic walls. 1947. Every point therefore in the intestine and skin absorbs, and thus every part is perforated with infinitely numerous holes or pores. Here they absorb air, there water. 1948. Every absorbent point of integument is drawn out as a tube towards the respiratory system, in order to let what it has absorbed become oxydized. These tegumentary prolongations into tubes are the _lymphatic vessels_. 1949. In all teguments there are necessarily lymphatic vessels, but more of them in the water-than the air-teguments. On that account the lymphatic vessels are much more numerous in the intestine than the skin. They are there called chyliferous, or lacteal vessels. 1950. The lymphatic vessels are the first of all vessels. Many animals, as perhaps the Acalephæ and Distomata, appear to have only this kind of vessel.