Page:The Zoologist, 4th series, vol 1 (1897).djvu/289

Rh submerged on the margins of Derwentwater and elsewhere at this period. Slight chemical changes produce greater or lesser degrees of hardness in the substance. Hence horny substances are not all alike hard, and the horny capsule of the worm is tolerably elastic; so that when kept in a moist condition it can be slightly expanded by the internal pressure exerted by the growing worm. But how can the worm grow? The chick can become no larger than the shell-surrounded yolk and albumen will permit, but when the young worm is hatched it will very probably be an inch in length. Let us see how this contingency is provided for. The following illustration will help to make the matter plain. If we took a small tube of gelatine, and placed within its cavity a tiny globule, we could secure the contents of the tube by drawing the two ends to a point. If now the globule could expand on the application of moisture, it must either burst its case, cause it to expand in the direction of its shortest diameter, or force open the ends of the tube. Now the egg-capsule of the worm can expand slightly, but not to a sufficient extent to allow the worm to reach full dimensions. Consequently the embryo gradually forces open the sealed extremities of the case, and thus paves we way for its ultimate escape, at the same time that it loosens its swaddling bands, and develops little by little into a perfect worm. While the beak of the embryo bird develops and hardens within the shell sufficiently to enable it to peck its way out of the calcareous covering, the worm has no such tool for Opening its prison-house, and so these other means must be provided for its escape.

It may occur to some observant reader that a condition analogous to this is found in the case of the dung-flies' eggs, which are deposited with their horn-like projections upwards. In both instances, if the eggs are removed from their moist lodging-place, they shrivel and become lifeless. Worms again are not quite alone in the possession of the power to extend the egg-case during incubation. Huber long ago observed the same fact in relation to the eggs of ants, and those of certain sawflies can similarly expand to meet the requirements of the growing grub within.

The question now arises—How does the worm lay its eggs? Although many careful observations have been made for the Zool. 4th ser. vol. I., June, 1897.