Page:Popular Science Monthly Volume 22.djvu/382

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has no peculiarities to indicate that the structure and habits of the adult are to be in any way strange or unusual; but after a time it finds its way, in some manner which has not been clearly made out, into the body of an holothurian, and this change in its habits is accompanied by a most marvelous change in its organization. It now has no shell, since the body-wall of its host affords ample protection, and, as it no longer needs to change its position in search of food, or to escape its enemies, its foot and specialized muscles have disappeared. Its organs of sense are wanting, and the nervous system is so rudimentary that no traces of it can be discovered. It has no need of organs to capture, masticate, or digest its food, since it sucks this, already digested, from the stomach of its host, and its digestive system has accordingly become reduced to a simple pouch, with only one opening—the mouth—and, as the whole surface of the body is bathed by the blood which is aërated in the respiratory organs of the holothurian, it has no need for gills, or heart, or blood-vessels, and, so far as our knowledge goes, these organs are entirely lacking.

PSM V22 D382 Cyclical reversion of evolutionary change.jpg
Fig. 3.

Of the highly specialized organs of a gasteropod only the simple stomach, the reproductive organs, and the slightly muscular body-wall remain, and no person who is not acquainted with the fact that young snails with spiral shells have been seen to come from its eggs would suspect that entoconcha is a mollusk.

Such cases, which modern research has proved to be by no means infrequent, show that the comparative study of adult animals can not furnish a complete key to their past history, and they also illustrate how little is to be hoped for from paleontology.

No one who accepts the doctrine of descent with modification, and is familiar with the embryology of the gasteropods, can doubt that, if we were able to trace back the pedigree of entoconcha, we should be led to a remote ancestor which was an ordinary gasteropod; not necessarily a species exactly like any we know, but a form with general gasteropod characteristics at least: nor can we doubt that, if we were able to study the embryology of this ancestral form, which we may represent by the letter A in diagram 3, we should find its life, from the egg to maturity, to be made up of a series of stages, a, b, c, d, e, f, etc., substantially like stages in the life of ordinary gasteropods, B and C. The relation between the entoconcha of the present day, D, and this gasteropod ancestor, is shown by the curved line to D. Start-