1953. This vascular system is therefore the pure remnant of the plant, and has as yet assumed no properly animal character, except that both its sets of vessels or tubes are self-substantial, and ramify, while in the plant they are only intercellular passages or non-ramified spiral vessels.
1954. In insects this system has been most perfectly evolved; there the air-tubes, being well parcelled out, are in great number, and run directly to the intestine and dorsal vessel, which is, as it were, only the trunk of the lymphatic vessels, or the chyliferous duct.
1955. Now, such a vascular system, merely oscillating, as it were, between intestine and skin, can be but persistent in animals, which express only the vegetable type of organization. If other significations be introduced, so also will this vascular system be otherwise evolved.
1956. The unclosed vascular system will be present along with an energetic antagonism between skin and intestine; with an antagonism that is almost suppressed, there will be none. There are then only cells, or there is only point-substance, as in the Infusoria, polyps, and Acalephæ or sea-nettles.
1957. The dermal vessels exist only in the air-insects, because in them nothing but air and water are engaged in conflict. Externally there is desiccated horn, internally mucous water.
1958. A perfectly unclosed vascular system appears to be developed only in animals which respire air. At least there are only genuine air-tubes and lymphatic vessels in such as breathe air; e. g., in the Mammalia, birds, reptiles and fishes, the latter set of vessels being probably not present in insects.
1959. Through the predominance of the air-process, as in insects, the mucus that is conveyed to it becomes so rapidly decomposed, that no more remains behind, for which a new vessel would be necessary.
1960. The galvanic process is at every instant annulled, and only renovated by a new afflux or supply. Here the galvanism does not subsist in itself as a peculiar and independent system.
b. Closed Vascular System.
1961. The unclosed vascular system is not yet self-substantial, because it is a cæcal eversion of the intestine or inversion of the skin, being itself only a ramified intestine and skin. Every system, however, attains its perfection, by being rendered independent of its origin. Thus the leaf is the spiral vessel that has become free, the root the cellular tissue in a like condition, the blossom the liberated vegetable trunk. The vascular system will therefore aspire also to the achievement of its blossom.