ANCESTRY OF LEPTUS. 161 



In seeking for the ancestry of our hypothetical Leptus among 

 the worms, we are at best groping iu tlie dark. We know of 

 no ancestral form among the true Anuelides, 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 pro- 

 vided 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. 



We may, however, as bearing upon this difficult question, cite 

 some remarkable discoveries of Professor Gauin, a Kussian 

 naturalist, on the early stages 

 of certain ichneumon parasites, 

 which show some worm fea- 

 tures in their embryonic devel- 

 opment. In a species of Platy- 

 gaster (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 l^^. Platygaster error, 



nucleus and nucleolus. Out of this cell (Fig. 193 A, a) arise 

 two other cells. The central cell (a) gives origin to the em- 

 bryo. 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\he sub- 

 division 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 

 (h). 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 iu a more 



