The structure of the head of centipedes is shown in part in Fig. 12, compare also Fig. 8. It will be seen that it differs much from that of the diplopods, though the mandibles (protomalæ) are homologous; they are divided into a cardo and stipes, thus being at least two-jointed.
The second pair of postoral appendages is in centipedes very different from the gnathochilarium of diplopods. As seen in Fig. 12 2, they are separate, cylindrical, fleshy, five-jointed appendages, the maxillary appendages of Newport, which are “connected transversely at their base with a pair of soft appendages” (c), the lingua of Newport. The third and fourth pair are foot-jaws, and we have called them malipedes, as they have of course no homology with the maxillipedes of Crustacea. The second pair of these malipedes, forming the last pair of mouth-appendages, is the poison-fangs (4), which are intermediate between the malipedes and the feet; Meinert does not allow that these are mouth-appendages.
Fig. 12.—Structure of a chilopod. A, Lithobius americanus, natural size. B, under side of head and first two body-segments and legs, enlarged: ant, antenna; 1, jaws; 2, first accessory jaw; c, lingua; 3, second accessory jaw and palpus; 4, poison-jaw. (Kingsley del.) C, side view of head (after Newport): ep, epicranium; l, frontal plate; sc, scute; 1, first leg; sp, spiracle.
The embryology of Geophilus by Metschnikoff shows plainly the four pairs of post-antennal appendages. The embryo Geophilus is hatched in the form of the adult, having, unlike the diplopods, no metamorphosis, its embryological history being condensed or abbreviated. But in examining Metschnikoff’s figures certain primitive diplopod features are revealed. The body of the embryo shortly before hatching is cylindrical; the sternal region is much narrower than in the adult, hence the insertions of the feet are nearer together, while the first six pairs of appendages begin to grow out before the hinder ones. Thus the first six pairs of appendages of the embryo Geophilus correspond to the antennæ, two pairs of jaws, and three pairs of legs of the larval Julus. These features appear to indicate that the chilopods may be an offshoot from the diplopod stem. The acquisition of a second pair of legs to a segment in diplopods, as in the phyllopod Crustacea, is clearly enough a secondary character, as shown by the figures of Newport in his memoir on the development of the Myriopoda (Pl. IV.). Thus the tendency in the Myriopoda, both diplopods and chilopods, is towards the multiplication of segments and the elongation of the body, while in insects the polypodous embryo has the three terminal segments of the abdomen well formed, these being, however, before hatching, partly atrophied, so that the body of insects after birth tends to become shortened or condensed. This indicates the descent of insects from ancestors with elongated polypodous hind-bodies like Scolopendrella. Korschelt and Heider suggest that the stem-form of myriopods was a homonomously jointed form like Peripatus, consisting of a rather large number of segments, but we might, with Haase, consider that the great number of segments which we now find indicates a late acquisition of this form.
The genital opening in chilopods is single, and situated in the penultimate segment of the body, as in insects. While recognizing the close relationship of the Myriopoda with the insects, it still seems advisable not to unite them into a single group (as Oudemans, Lang, and others would do), but to regard them as forming an equivalent class. On the other hand, when we take into account the form and structure of the head, antennæ, and especially the shape of the first pair of mouth-appendages, being at least two-jointed in both groups, we think these characters, with the homonomously segmented body behind the head, outweigh the difference in the position of the genital outlet, important as that may seem. It should also be taken into account that while insects are derived from polypodous ancestors, no one supposes, with the exception of one or two authors, that these ancestors are the Myriopoda, the latter having evidently descended from a six-legged ancestor, quite different from that of the Campodea ancestor of insects.[[4]]
In regard to the sexual openings of worms, though their position is in general in the anterior part of the body, it is still very variable, though, in general, paired. In the oligochete worms the genital zone, with the external openings, is formed by the segments lying between the 9th and 14th rings, though in some the genital organs are situated still nearer the head. The myriopods, which evolved from the worms earlier than insects, appear to have in their most primitive forms (the Diplopoda) retained this vermian position of the genital outlets. In the later forms, the chilopods, the genital openings have been carried back to near the end of the body, as in insects. From observations made by three different observers on the freshly hatched larva of the Julidæ, it appears that the ancestral diplopods were six-footed, or oligopod, the larva of Pauropus (Fig. 13) approaching nearest to our idea of the ancestral myriopod, which might provisionally be named Protopauropus.
Relations of the Symphyla to Insects.—Opinions respecting the position of the Symphyla, represented by Scolopendrella (Fig. 14), are very discordant. By most writers since Newport, Scolopendrella has been placed among the myriopods. The first author, however, to examine its internal anatomy was Menge (1851), who discovered among other structures (tracheæ, etc.) the silk-glands situated in the last two segments, and which open at the end of each cercus. He regarded the form as “the type of a genus or family intermediate between the hexapod Lepismidæ and the Scolopendridæ.”
Fig. 13.—Pauropus huxleyi, much enlarged. A, enlarged view of head, antennæ, and first pair of legs (original). B, young.—After Lubbock. C, longitudinal section of Pauropus huxleyi, ♂: a, brain; b, salivary gland; k, mid-intestine; g, rectum; h, ventral nerve-cord; c, bud-like remnants of coxæ; d, penis; e, vesicula seminalis; f, ductus glandularis; i1, divisions of testes.—After Kenyon.