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This fact sustains the view of Gegenbaur that at first the tracheæ formed two simple tubes in the body-cavity, and that the primary office of these tubes was for lightening the body, and that their function as respiratory tubes was a secondary one.
We find the vascular system in this very simple form to-day in the Bryozoa, Rotatoria, Nematoda, and other lower Vermalia. The first step in the improvement of this primitive vascular system is the formation of larger canals or blood-conducting tubes. The spaces filled with blood, the relics of the primary body-cavity, receive a special wall.
Their earliest source is the primary body-cavity, the simple space between the two primary germinal layers, which is either a relic of the segmentation-cavity, or is a subsequent formation. Amoeboid planocytes, which migrate from the entoderm and reach this fluid-filled primary cavity, live and multiply there, and form the first colourless blood-cells.
The coeloma or body-cavity has some very important and distinctive features in the Amphioxus. The embryology of it is most instructive in connection with the stem-history of the body-cavity in man and the other vertebrates.
It is true that the Amphibia have two lungs, right and left. Ventral and pectoral walls are removed. The greater part of the body-cavity is taken up with the liver, from the middle part of which the caecum and the vermiform appendix protrude. Immediately behind the single structure of the thyroid gland a median groove, the rudiment of the trachea, is detached from the gullet.
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