[Illustration: FIG. 28.] [Illustration: FIG. 30.] _OPTICAL SECTIONS._ [Illustration: FIG. 25.] [Illustration: FIG. 27.] [Illustration: FIG. 29.] [Illustration: FIG. 31.] THE DEVELOPMENT OF A WOODLOUSE (_Porcellio scaber_), AFTER ROULE. Figs. 24, 26, 28, 30, are Surface Views, and figs. 25, 27, 29, 31, which indicate slightly later stages respectively than the others, are of egg seen in Optical Section. The body next alters somewhat in shape and the three divisions of the intestine approach one another (see fig. 32) previous to their junction. As may be imagined during these processes the food-yolk has gradually been used up and the space which it occupied taken by the internal organs, which we have mentioned. [Illustration: FIG. 32.--EMBRYO OF THE WOODLOUSE SHOWING THE THREE DIVISIONS OF THE INTESTINE SEPARATELY DEVELOPED (_Porcellio scaber_), AFTER ROULE.] [Illustration: FIG. 33.--EMBRYO OF THE WOODLOUSE SHOWING TRACES OF THE SEGMENTS (_Porcellio scaber_), AFTER ROULE.] [Illustration: FIG. 34.--AN EMBRYO WOODLOUSE READY TO BE HATCHED (_Porcellio scaber_), AFTER ROULE.] In the last stages of the development the appendages become larger still, the _heart_ makes its appearance, segmentation of the body is completed, and except that the seventh pair of walking legs are as yet rudimentary the woodlouse is completed. It is only after hatching that the pair of legs mentioned, attain to their normal length. The process of segmentation of the egg and the formation of its layers lasts about a fortnight, while the completion of the development proceeds much more rapidly, for another three weeks bring it to an end. After the first moult or change of skin the last pair of walking legs makes its appearance, and Mr. James B. Casserley [whose work one of us (75) has described elsewhere] found when keeping a number of the common pill-woodlouse (_Armadillidium vulgare_) in captivity that his specimens did not subsequently change their skins more than once in the six months during which he had them under observation. He also noted that the crustaceans go on growing after they are sexually mature. As his specimens grew older, Mr. Casserley noticed that their colour became darker, and a curious point recorded by him is that two examples of the same age may change their skins at the same time, and while one may have afterwards nearly twice as many markings, on the other very few at all will be seen. The time required for the growth of a woodlouse from the size of a pin's head to that of an adult example--say three-quarters-of-an-inch long--must be fairly considerable, taking into account the fact that any appreciable increase in size can only occur at a moult and Mr. Casserley's observations as to the infrequency of the process in _Armadillidium vulgare_. (See p. 14.)