[Illustration: FIG. 66.--The Lamprey--Petromyzon.]
[Illustration: 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.
[Illustration: FIG. 68.--Diagrammatic cross-section of lamprey larva.
_n_, Nerve cord; _ch_, notochord; _ar_, artery; _v_, vein; _g_, reproductive body; _d_, alimentary canal.]
[Illustration: FIG. 69.--Mouth of lamprey.]
We have already observed that the true fishes, to which we must now direct our attention, differ from the round-mouths in several important characters. They possess two pairs of extremities, the pectoral and pelvic fins; they have a pair of nostrils; there is also a well-developed skull, which includes a series of cartilaginous or bony arches situated in the wall of the gut and between the successive gill clefts. These branchial arches bear a certain resemblance to the basket-work arrangement of cartilage in the round-mouths, but for various reasons are not regarded as having been derived from the latter. It is from the first pair of these arches that the jaws are formed, organs which make their first appearance in the lower fishes. The skeleton shows great development in other directions. The notochord is present in its primitive condition during the earlier stages of development, but it becomes surrounded, and in many cases largely suppressed, by the portions of the vertebræ. Each vertebra consists of an upper and a lower portion, the upper forming an arch round the nerve cord and the lower bearing lateral processes or ribs. In the higher forms the two portions become united round the notochord, and the resulting vertebra may encroach inwards until it becomes solid, the notochord then remaining only as a series of small pieces of cartilage between the successive units of the vertebral column. There is also, of course, a skeleton in connection with the limbs, but this does not yet correspond in detail to that of the other classes of vertebrates. The brain is much more highly developed than in the round-mouths; in many forms, particularly, there is a considerable development of the cerebral hemispheres, a portion of the fore brain, and the seat of the higher intelligence. The eyes and ears show the same main features as in the higher groups. The ear has three semicircular canals, the same number as in man, as against two in the lamprey and one in the hag. Fishes are possessed of a peculiar 'sixth sense,' the organs for which are situated in two lines running along the sides of the body, the latter forming a familiar feature of a cod or whiting. The nature of this sense is not definitely known, but it appears to be of the nature of a very refined appreciation of wave motions in the water. It is probably by means of these 'lateral line' sense organs, for instance, that fishes are so easily able to avoid obstacles when swimming in the dark.