The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

members of that group. Secondly, it has a spinal cord down its back, which, like that of a vertebrate, is a hollow tube traversed by a continuous narrow canal. Thirdly, although its digestive tube runs straight from mouth to anus, there opens out of it a blind pouch which seems to be the homologue of our liver. The blood that has supplied the intestine is carried by a portal vein to this pouch and there traverses a second system of capillaries, a portal circulation, such as all vertebrata have; and although the liver of an adult vertebrate is not a simple pouch, it nevertheless develops in the embryo from a pouch that grows out from the intestine like the liver-pouch of Amphioxus. And another most important point of resemblance is a structure called the notochord. Amphioxus has no vertebrae, but it has a stiff, elastic tod, lying along the whole length of the body, between the spinal cord and the alimentary canal. This notochord, like our backbones, is the mechanical axis of the body on which the muscles play. It is made of a highly characteristic kind of tissueits cells have large spaces inside them, containing water under pressure, and it is surrounded by a thin, elastic membrane secreted by the cells, so that its rigidity and elasticity are like the rigidity and elasticity of a rubber tube filled with water under pressure. Now, in the embryos of all vertebrated animals a notochord precisely similar to the notochord of Amphioxus appears. In the lampreys and their allies, and in a few fish, this persists and the backbone forms round it and encloses it, but in all other vertebrata it is supplanted during later development by a true backbone that owes its rigidity to lime-salts and its elasticity to binding ligaments.

The notochord is really the distinctive character of the phylum that we are now examining, and which is often called, for this reason, the Chordata. In the humblest members of the phylum, such as Amphioxus, the notochord is retained throughout life. In the higher forms, in all the true vertebrata, it is present in the embryo, but becomes replaced more or less completely by a better axial skeleton—stronger and as flexible, and at the same time affording a protection for the spinal cord—the column of jointed vertebre.

Another still more remarkable affiliation to the vertebrate phylum in recent years has been that of the sea-squirts and their allies. Sea-squirts are common enough on rocky shores, where they occur, often in

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THE SCIENCE OF LIFE

CHAPTER f

large masses, growing fixed to rocks and the like. An individual sea-squirt is a roughly cylindrical, gelatinous, translucent object, often several inches in length; at the unattached end there are two holes from which, when it is touched, it ejects with considerable force two jets of water. On examining the creature more closely we find that its gelatinous exterior is not part of the animal itself but a garment that it secretes around its body. When the seasquirt is unmolested, a current of water passes slowly in at one of the openings and out at the other; on the way through the body it traverses a gas-mantle filter or pharynx, like that of Amphioxus, and there yields the suspended particles upon which the animal subsists. The whole adult life of the sea-squirt is spent in the same place, and its only outwardly visible sign of animation is the slight stir in the surrounding water produced by its slow, indolent feedingcurrent—except on rare occasions when, alarmed by the approach of a hungry fish, it contracts for a while into an unattractive gelatinous blob, squirting water out of itself in two violent jets.

There is nothing very like a chordate in the anatomy or mode of life of an adult seasquirt, nothing to suggest a nearer cousinship to Homo sapiens than that which a lobster or spider might claim—except that its food filter is highly suggestive of that of Amphioxus. But when its life-cycle is examined the case is altered. It is upon the anatomy of its early life that its inclusion among the chordata is based. The egg of a sea-squirt develops into a lively little creature like a minute tadpole, which swims along by means of a powerful muscular tail. This larva is quite obviously a chordate. In its tail it has a notochord like the notochord of Amphioxus, and the mechanics of swimming are very similar in both forms ; it has a tubular nerve-cord running along its back. But in a little while it settles down on a rock, sitting on its head and fastening itself by means of sticky horns that it possesses there. It now absorbs its tail and notochord, and its nervous system (since it no longer has anything very much to think about) dwindles to a single rounded ganglion with a few radiating branches ; at the same time it loses its eyes and other special sense-organs.

The class to which the sea-squirt belongs, the Tunicata, includes a number of other curious forms. Many of them are able to reproduce themselves by sprouting out buds ; in this way they give rise to extensive colonies of individuals embedded in a