Fig. 66.—The Lamprey—Petromyzon.
Fig. 67.—The Hag-fish—Myxine.
There remain, even after the most thorough investigation of Balanoglossus, the lancelet, and the round-mouths, some questions with regard to the origin of the vertebrates that are still unanswered. It must, however, be regarded as an extremely fortunate circumstance that representatives have come down to us of the three ancient groups to which these three types respectively belong. This is the more fortunate in that the groups in question are known only from their few living members, a circumstance which is of course easily accounted for by the absence of any hard parts capable of being preserved as fossils. From this point onwards there is a skeleton, and we are consequently enabled to draw valuable information from fossils. Partly in consequence of this, the Evolution chain from this point onwards is much more complete than the portion that we have dealt with thus far.
Fig. 68.—Diagrammatic cross-section of lamprey larva.
n, Nerve cord; ch, notochord; ar, artery; v, vein; g, reproductive body; d, alimentary canal.
Fig. 69.—Mouth of lamprey.