Page:Popular Science Monthly Volume 45.djvu/547

Rh bell; later, it took a strange shape, which was not inappropriately compared to a lectern. M. Chabry even designated it by the Latin name pluteus, which means pulpit. As the time for this change of form approaches, there can be seen appearing in the tissues of the young larva a kind of calcareous needles, called spicules, the form and disposition of which are identical in all individuals of the same species. These spicules are composed of the carbonate of lime which the larva finds in the sea water, and which it absorbs as the roots of a plant absorb the potash contained in the soil. This lime traverses the tissues of the larva and collects for a time in them before settling in the half-crystalline figure of the spicules. It may be remarked that although they present a regular arrangement in the larva, the spicules have no relation, at least in the beginning, with the external form or the shape of the organs of the animal.

M. Chabry asked what would happen if he tried by raising the larvæ in water destitute of lime to prevent the formation of the spicules. The experiment was not without difficulties. It was necessary to prepare artificially a limeless sea water. With all the pains M. Chabry could take, in the light of the best analysis, the larvæ perished in the artificial water as soon as they were hatched. He then tried diminishing by degrees the proportion of lime in the natural water. This lime was the sulphate, and the experiment was directed, in order to prevent too radically changing the water, to substituting another base for calcium. Sodium was taken, because, it being already very abundant in the water, the slight addition of it which it would be necessary to make to replace the lime could not have any great influence. The results were very plain. Without any mixture of lime in the water, the just-hatched larvæ were arrested in their development and died in a few hours. If the elimination of calcium is not pushed to its extreme limits, and only a fifteenth part of the already very slight quantity contained in sea water is left, the larvæ will not be for forty hours distinguishable from those which are developed in normal water. At the end of that time the spicules should appear while the larva is assuming the form of the pluteus. But in water containing only a fifteenth of the normal calcium this change is not effected. Twenty hours later, in the sixtieth hour of their lives, the larvæ are still in the same condition, while those in normal water have spicules already branched, and their having taken the form of the pluteus is marked both by their shape and by the division of their intestine into distinct regions. The larvæ deprived of lime first exhibit this modification of the intestine toward the ninetieth hour, but they have no spicules and have not become pluteus. Their external form has therefore been profoundly affected by some change that has been introduced into the inner