The sexes are separate, but traces of a previous hermaphrodite condition seem to persist. Henceforward in the vertebrate group the sexes are always separate. A character of the sex organs which is to be regarded as primitive is that they are unconnected with the excretory system, whereas in the higher vertebrates the two systems are always strangely interconnected. As in all the higher types, there is but one pair of male or female reproductive glands. Finally, the round-mouths differ markedly from the lancelet in the structure of the skin. In the latter animal the skin is composed of a single layer of cells. In the former it consists of an epidermis, some three or four layers thick, and an underlying cuticle or 'true skin'; in other words, the skin has the same general structure as that of the higher types. The lamprey and the hag are illustrated in Figs. 66 and 67, and some of the main anatomical features of the group are shown in Figs. 63, 68, and 69. [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.]