Fig. 33.—Embryo of Julus (after Newport).
Among Centipedes the development of Julus has been described by Newport.[24] The first period, from the deposition of the egg to the gradual bursting of the shell, and exposure of the embryo within it, which, however, remains for some time longer in connection with the shell, lasts for twenty-five days. The segments of the body, originally six in number, make their appearance on the twentieth day after the deposition of the egg, at which time there were no traces of legs. The larva, when it leaves the egg, is a soft, white, legless grub (Fig. [33]), consisting of a head and seven segments, the head being somewhat firmer in texture than the rest of the body. It exhibits rudimentary antennæ, but the legs are still only represented by very slight papilliform processes on the undersides of the segments to which they belong.
As already mentioned, it is possible that at one time the vermiform state of the Homomorphous insects—which, as we have seen, is now so short, and passed through at so early a stage of development—was more important, more prolonged, and accompanied by a more complete condition of the internal organs. The compression, and even disappearance of those embryonal stages which are no longer adapted to the mode of life—which do not benefit the animal—is a phenomenon not without a parallel in other parts of the animal or even of the vegetable kingdom. Just as in language long compound words have a tendency to concision, and single letters sometimes linger on, indicating the history of a word, like the “l” in “alms,” or the “b” in “debt,” long after they have ceased to influence the sound; so in embryology useless stages, interesting as illustrations of past history, but without direct advantage under present conditions, are rapidly passed through, and even, as it would appear, in some cases altogether omitted.
Fig. 34.—Colony of Bougainvillea fruticosa, natural size, to the underside of a piece of floating timber (after Allman).
For instance, among the Hydroida, in the great majority of cases, the egg produces a body more or less resembling the common Hydra of our ponds, and known technically as the “trophosome,” which develops into the well-known Medusæ or jelly-fishes. The group, however, for which Prof. Allman has proposed the term Monopsea,[25] and of which the genus Ægina may be taken as the type, is, as he says, distinguished by the absence of a hydriform stage, “the ovum becoming developed through direct metamorphosis into a medusiform body, just as in the other orders it is developed into a hydriform body.” Fig. [34] represents, after Allman, a colony of Bougainvillea fruticosa of the natural size. It is a British species, which is found growing on buoys, floating timber, &c., and, says Allman,[26] “when in health and vigour, offers a spectacle unsurpassed in interest by any other species—every branchlet crowned by its graceful hydranth and budding with Medusæ in all stages of development (Fig. [35]), some still in the condition of minute buds, in which no trace of the definite Medusa-form can yet be detected; others, in which the outlines of the Medusa can be distinctly traced within the transparent ectothèque (external layer); others, again, just casting off this thin outer pellicle, and others completely freed from it, struggling with convulsive efforts to break loose from the colony, and finally launched forth in the full enjoyment of their freedom into the surrounding water. I know of no form in which so many of the characteristic features of a typical hydroid are more finely expressed than in this beautiful species.”
