Page:Popular Science Monthly Volume 8.djvu/199

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187
PROGRESSION AND RETROGRESSION.

water snail. We need not follow him into details. It will be enough for our purpose to note that from a "mulberry mass"—the egg after segmentation of the yelk—there comes a sort of hemispherical cup. The mouth of the cup changes from a circle to a long slit, and the edges of the slit unite except at one point. The embryo has now taken on the molluscan type. The aperture along the line of the slit is the opening to the sac, the mouth to the coming snail. The line along which the approximated sides of the cup have united is in the trend of a plane which divides the body into right and left sides, equal

PSM V08 D199 Embryonic snail.jpg
Fig. 4.—Symmetry. Embryotic Snail: m, mouth; ma, mantle; c, creeping disk; in, intestine; h, heart (auricle and ventricle in line with the intestinal tube); r, remnants of yolk-cell.

and similar. The mantle has begun to form, and as a sort of cap it covers the part of the body opposite the mouth. The intestine begins in a little depression under the mantle and in line with the mouth and stomach. This depression is elongated, becomes a tube, and opens into the stomach. A few days later, traces of a heart appear as two pulsating, globular sacs, placed end to end (Fig. 5).

If development were arrested at this stage, our snail would be bi-symmetrical, and, if it had a shell, the shell would be in two equal valves, right and left. But development goes on, and now every step is a departure from right and left symmetry. First, the intestine gets a, twist. Other organs are quick to follow. Even the heart moves askance. The two chambers which, a while before, were placed end to end in line with the axis of the body, begin to change position. The receiving chamber moves obliquely to the right and downward, the distributing chamber to the left and upward. The right fold of the mantle spreads rapidly; the left, not at all. The right side of the body grows rapidly; the left remains almost stationary. The right valve of the shell grows rapidly, and twists over with the inclosed body; the left is completely aborted. Now, it is a very significant fact that the only parts which do not share this one-sided overgrowth are the head and creeping disk; and these are the parts which, not being covered by the mantle, do not become incased in the shell. Exposed to the water or the air equally on both sides, they retain their bilateral symmetry.