1971. The artery contains the whole body, lung and intestine, in short, the whole animal, (whereas it was previously dispersed in two vessels,) in a fluid state within itself. From the artery therefore nutrition will directly take place; from it the animal will be formed.
1972. The air or respiratory vessels may be viewed as arteries carried to the very extreme. In the lung the arterial system has attained its highest purity, the oxygen only, without the indifferent substance, being contained therein. The trachea is the roughest artery—Arteria aspera.
1973. The arterial system, in accordance with its signification, makes its first appearance in water, because the aquatic mode of respiration is less energetic, and thus the mucus is more feebly decomposed. The Mollusca, snails, and many worms have a perfect arterial system. The branchiæ do not, like respiratory tubes, pass into the body, as in insects; but there are vessels which take up the oxygen and convey it into the body.
1974. As lung and absorbent belong to the pure air or the pure water, so do artery and vein to the water combined with the air. The two former are therefore present only where aerial respiration occurs, and the two latter where water merely is respired.
1975. The last system is only present in animals, in so far as they are aquatic in their habits.
1976. Insects, as being purely aerial animals, have therefore arteries and veins only so long as they are in the larva or worm-like condition, and as flies or perfect insects may continue to live without them. On the contrary, the purely aquatic animals appear enabled to live without true respiratory and absorbent vessels. It in fact appears, that lymphatic as well as respiratory vessels are wanting in the molluscs, snails, and worms, since the water directly bathes or washes the arteries.
1977. Animals with both systems of vessels, the unclosed and closed, must be more perfect in structure, and must at once combine worm and insect in themselves. They are insects from having absorbent and respiratory vessels, but worms as having arteries and veins.
b. Veins.
1978. The veins are developed as mucus-vessels at the intestinal extremities of the arteries, which absorb the arterial mucus (blood), after it has deposited its air on the tegumentary substance, just as the lymphatic vessels absorb their fluid from the intestine or any other part of the body.
1979. As the artery is a respiratory vessel that has become self-substantial, so is the vein a similarly conditioned, and dismembered lymphatic vessel. In the one it is the lung, in the other the intestine, that has become the free vascular system. But in the proper vascular system both lung and intestine are repeated, the former as artery, the latter as vein.