When we come to the singular creatures of which Pentastoma and Linguatula are the type, we have the most striking approximation to the worms in external form, but these are induced evidently by their parasitic mode of life. They lose the rudimentary jointed limbs which some (Linguatula especially) have well marked in the embryo, and from being oval, rudely mite-like in form, they elongate, and only the claws or simple curved hooks, like those of young tape worms, remain to indicate the original presence of true jointed legs.
In seeking for the ancestry of our hypothetical Leptus among the worms, we are at best groping in the dark. We know of no ancestral form among the true Annelides, nor is it probable that it was derived from the intestinal worms. The only worm below the true Annelides that suggests any remote analogy to the insects is the singular and rare Peripatus, which lives on land in warm climates. Its body, not divided into rings, is provided with about thirty pairs of fleshy tubercles, each ending in two strong claws, and the head is adorned with a pair of fleshy tubercles. It is remotely possible that some Silurian land worm, if any such existed, allied to our living Peripatus, may have been the ancestor of a series of types now lost which resulted in an animal resembling the Leptus.
192. Platygaster error.
We may, however, as bearing upon this difficult question, cite some remarkable discoveries of Professor Ganin, a Russian naturalist, on the early stages of certain ichneumon parasites, which show some worm features in their embryonic development. In a species of Platygaster (Fig. 192, P. error of Fitch), which is a parasite on a two-winged gall fly, the earliest stage observed after the egg is laid is that in which the egg contains a single cell with a nucleus and nucleolus. Out of this cell (Fig. 193 A, a) arise two other cells. The central cell (a) gives origin to the embryo. The two outer ones multiply by subdivision and form the embryonal membrane, or "amnion," which is a provisional envelope and does not assist in building up the body of the germ. The central single cell, however, multiplies by the subdivision of its nucleus, thus building up the body of the germ. Figure 193 B, g, shows the yolk or germ just forming out of the nuclei (a) and b, the peripheral cells of the blastoderm skin, or "amnion." Figure 193 C shows the yolk transformed into the embryo (g), with the outer layer of blastodermic cells (b). The body of the germ is infolded, so that the embryo appears bent on itself. Figure 193 D shows the embryo much farther advanced, with the two pairs of lobes (md, rudimentary mandibles; d, rudimentary pad-like organs, seen in a more advanced stage in E), and the bilobate tail (st). Figure 194 (m, mouth; at, rudimentary antennæ; md, mandibles; d, tongue-like appendages; st, anal stylets; the subject of this figure is of a different species from the insect previously figured, which, however, it closely resembles) shows the first larva stage after leaving the egg. This strange form, the author remarks, would scarcely be thought an insect, were not its origin and farther development known, but rather a parasitic Copepodous crustacean, whence he calls this the Cyclops-like stage. In this condition it clings to the inside of its host by means of its hook-like jaws (md), moving about like a Cestodes embryo with its well known six hooks. The tail moves up and down, and is of but little assistance in its efforts to change its place. Singularly enough, the nervous, vascular, and respiratory systems (tracheæ) are wanting, and the alimentary canal is a blind sac, remaining in an indifferent, or unorganized state. How long it remains in this state could not be ascertained.
193. Development of Platygaster.
194. First Larva of Platygaster.
