Page:Popular Science Monthly Volume 19.djvu/400

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which mark the phases of ascidian life-history. Thus it is matter of sober, natural-history fact that a sea-squirt larva, of all invertebrate animals, is the only being that possesses organs and parts proper to the young vertebrate or to the adult form of one lower vertebrate in

PSM V19 D400 Sea squirt and its development.jpg
Fig. 16.—Sea-Squirt. Fig. 17.—Development of Sea-Squirt.

particular. This adult is the little fish known as the lancelet, which, in the relative simplicity of its organization, makes a nearer approach to the poor or sea-squirt relations of the vertebrates than any other fish.

The fact of vertebrate and sea-squirt relationship is worth dwelling upon, because the topic unquestionably presents one with a common point of view, whence a survey of the higher development, evolution, and progress of the vertebrates, and a view of the degeneracy and retrogression of the sea-squirts, may best be obtained. Reveling in the freedom of its early life, the larval sea-squirt—presenting, as already noted, a striking resemblance to the tadpole of the frog, in its backbone, its nerve-system, and its breathing-sac, or modified throat—ultimately settles down. Like the youthful barnacle somewhat, the young sea-squirt attaches itself to a stone or shell by the suckers with which nature has provided its head. Then succeeds the disappearance of the tail, with its backbone and its nerve-cord, and the body itself soon assumes the sac-like shape that betokens the mature ascidian character. The outer skin becomes tough and leathery, and develops the cellulose which, by biological right, we should expect to find in plants alone. Then succeeds the fuller formation of the gill-sac or breathing-chamber, and of its neighbor compartment, which receives the effete water of respiration to be ejected by the second mouth of the sac like