The Evolution of Man — Volume 2 by Ernst Heinrich Philipp August Haeckel
page 26 of 417 (06%)
page 26 of 417 (06%)
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The whole of the long tube that runs along the ventral side of the
alimentary canal and contains venous blood may be called the "principal vein," and may be compared to the ventral vessel in the worms. On the other hand, the long straight vessel that runs along the dorsal line of the gut above, between it and the chorda, and contains arterial blood, is clearly identical with the aorta or principal artery of the other vertebrates; and on the other side it may be compared to the dorsal vessel in the worms. (FIGURE 2.214. Transverse section of a young Amphioxus, immediately after metamorphosis, through the hindermost third (between the atrium-cavity and the anus). FIGURE 2.215. Diagram of preceding. (From Hatschek.) A epidermis, B medullary tube, C chorda, D aorta, E visceral epithelium, F subintestinal vein. 1 corium-plate, 2 muscle-plate, 3 fascie-plate, 4 outer chorda-sheath, 5 myoseptum, 6 skin-fibre plate, 7 gut-fibre plate, I myocoel, II splanchnocoel, I1 dorsal fin, I2 anus-fin.) 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. As we have already seen (Chapter 1.10), in these the two coelom-pouches are divided at an early stage by transverse constrictions into a double row of primitive segments (Figure 1.124), and each of these subdivides, by a frontal or lateral constriction, into an upper (dorsal) and lower (ventral) pouch. These important structures are seen very clearly in the trunk of the amphioxus (the latter third, Figures 2.212 to 2.215), but it is |
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