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.
1951. Lymphatic vessels are present in the skin, only in so far as it has resigned the respiratory function to special organs.
Meaning of the Unclosed Vascular System.
1952. The action of this unclosed vascular system is wholly similar to the motion of sap and air in the plant, there being only an ascent of the first, and a fall or descent of the last. In the absorbents the sap ascends out of the root (intestine) into the leaves (branchiæ); in the respiratory vessels the air descends from the foliage (skin) to the intestine and the whole body of cells.