But you will say he is quite different in having a gristly cord. True--but we shall find that even this does not give us a sharp line of division. By looking carefully upon the seaweed and rocks just beyond low tide, we may often find some curious small creatures upon them, called Sea-Squirts or Ascidians (B, Fig. 2).[5] These creatures are shaped very like double-necked bottles, and they stand fixed to the rock with their necks stretching up into the water. Through one neck (_m_) they take water in, and after filtering it through a kind of net so as to catch the microscopic animals in it and taking the air out of it, they send it out through the other neck, thus gaining the name of sea-squirts. So far, they are certainly boneless animals. But they were not always stationary, as you see them fixed to the rock. In their babyhood they were tiny swimming creatures with tails (A and a), and in the tail was a gristly cord (_r_), with a nerve cord (_nv_) above it, like those we find in the lancelet. For this reason we were obliged to pass them by among the lower forms of life, because, having this cord (_r_), they did not truly belong to the animals without backbones; and yet now we can scarcely admit them here, because when they are grown up they are not backboned animals. They belong, in fact, to a kind of “No Man’s Land,” behaving in many ways like the lancelet when they are young, as if they had once tried to be backboned; and yet they fall back as they grow up into invertebrate animals. So we begin to see that there may have been a time when backbones had not gained quite a firm footing, and our lancelet, with his friends the sea-squirts, seems to lie very near the threshold of backbone life. And now that we are once started fairly on our road, let us turn aside before beginning the history of the great fish-world and pay a visit to a little creature whose name, at least, we all know well, and which stands half-way between the lancelet and the true fish. This is the Lamprey, represented by two kinds; the large Sea-Lamprey, caught by the fishermen for bait as it wanders up the rivers to lay its eggs, and the true River-Lamprey or Lampern, which rarely visits the sea. What country boy is there who has not hunted in the mud of the rivers or streams for these bright-eyed eel-like fish, with no fins, and a fringe on back and tail? If you feel about for them in the mud they will often come up clinging to your hand with their round sucker-mouth, while the water trickles out of the seven little holes on each side of their heads. The small river-lampreys do not hurt in the least as they cling, though the inside of their mouth is filled with small horny teeth. But the larger sea-lamprey uses these teeth as sharp weapons, scraping off the flesh of fish for food as he clings to them. [Illustration: Fig. 3. Figure of a full-grown Lamprey[6] and of the young Lamprey, formerly called Ammocœtes.