In 1873[[5]] the writer referred to this form as follows: “It may be regarded as a connecting link between the Thysanura and Myriopoda, and shows the intimate relation of the myriopods and the hexapods, perhaps not sufficiently appreciated by many zoölogists.”
In 1880 Ryder regarded it as “the last survival of the form from which insects may be supposed to have descended,” and referred it to “the new ordinal group Symphyla, in reference to the singular combination of myriopodous, insectean, and thysanurous characters which it presents.[[6]]”
Fig. 14.—Scolopendrella immaculata, from above,—after Lang; also from beneath, the genital opening on the 4th trunk-segment: sac, eversible or coxal sac; an, anus; c, cereopod; v, vestigial leg.—After Haase, from Peytoureau. A B C, head and buccal appendages of Scolopendrella immaculata: A, head seen from above; cl, clypeus. B, head from beneath; l, first pair of legs; mx, 1st maxilla; mx1, 2d maxilla; t, “labial plates” of Latzel, labium of Muhr. C, 1st maxilla; l, lacinia; g, galea; p, rudiment of the palpus.—After Latzel. D, end of the body: p11, eleventh, p12, twelfth undeveloped pair of legs; p13, modified, vestigial legs, bearing tactile organs (so); sg, cercopod, with duct of spinning gland, dg; cd, eversible or coxal gland; h8s, coxal spur of the 11th pair of legs.—After Latzel from Lang.
Wood-Mason considered it to be a myriopod, and “the descendant of a group of myriopods from which the Campodeæ, Thysanura, and Collembola may have sprung.” We are indebted to Grassi for the first extended work on the morphology of Scolopendrella (1885). In 1886 he added to our knowledge facts regarding the internal anatomy, and gives a detailed comparison with the Thysanura, besides pointing out the resemblances of Scolopendrella to Pauropus, diplopods, chilopods, as well as Peripatus.
Fig. 15.—Section of Scolopendrella immaculata: œ, œsophagus; oe. v, œsophageal valve entering the mid-intestine (“stomach”); i, intestine; r, rectum; br, brain; ns, abdominal chain of ganglia; ovd, oviduct; ov, ovary; s. gl, silk-gland, and op, its outer opening in cercus, ur. t, urinary tube; cg, coxal glands or blood-gills.—Author del.
In 1888 Grassi expressed his view as to the position of the Symphyla, stating that it should not be included in the Thysanura, since it evidently has myriopod characters; these being the supraspinal vessel, the ventral position of the genital glands; the situation of the genital opening in the fourth segment of the trunk, its ganglionic chain being like that of diplopods, its having limbs on all the segments, etc. On the other hand, Grassi has with much detail indicated the points of resemblance to the Thysanura. The principal ones are the thin integument, the want of sympathetic ganglia, the presence of a pair of cephalic stigmata, like that said to occur in certain Collembola, and in the embryo of Apis; two endoskeletal processes situated near the ventral fascia of the head; the epicranial suture also occurring in Thysanura, Collembola, Orthoptera, and other winged insects, and being absent in diplopods and chilopods. He also adds that the digestive canal both in Symphyla and Thysanura is divided into three portions; the malpighian tubes in Thysanura present very different conditions (there being none in Japyx), among which may be comprised those of Scolopendrella. In both groups there is a single pair of salivary glands. The cellular epithelium of the mid-intestine of Scolopendrella is of a single form as in Campodea and Japyx. The fat-body, dorsal vessel, with its valves and ostia, are alike in the two groups, as are the appendages of the end of the abdomen, the anal cerci (cercopoda) of Scolopendrella being the homologues of the multiarticulate appendages of Lepisma, etc., and of the forceps of Japyx. In those of Scolopendrella, we have found the large duct leading from the voluminous silk-gland, a single large sac extending forwards into the third segment from the end of the body (Fig. 15, s. gl). Other points of resemblance, all of which he enumerates, are the slight differences in the number of trunk-segments, the presence in the two groups of the abdominal “false-legs” (parapodia), the dorsal plate, and the mouth-parts. As regards the latter, Grassi affirms that there is a perfect parallelism between those of Scolopendrella and Thysanura. To this point we will return again in treating more especially of those of the Symphyla. Finally, Grassi concludes that there is “a great resemblance between the Thysanura and Scolopendrella.” He, however, believed that the Symphyla are the forerunners of the myriopods, and not of the insects, his genealogical tree representing the symphylan and thysanuran phyla as originating from the same point, this point also being, rather strangely, the point of origin of the arachnidan phylum.
Haase (1889) regarded Scolopendrella as a myriopod, and Pocock (1893) assigned the Symphyla to an independent class, regarding Scolopendrella as “the living form that comes nearest to the hypothetical ancestor of the two great divisions of tracheates.” Schmidt’s work (1895) on the morphology of this genus is more extended and richly illustrated than Grassi’s, his method of research being more modern. He also regards this form as one of the lower myriopods.
In conclusion, it appears to us that, on the whole, if we throw out the single characteristic of the anteriorly situated genital opening, the ovarian tubes being directed toward the end of the body (Fig. 15, ovd, ov), there is not sufficient reason for placing the Symphyla among the Myriopoda, either below or near the diplopods. This is the only valid reason for not regarding Scolopendrella as the representative of a group from which the insects have descended, and which partly fills the wide abyss between Peripatus and insects. With the view of Pocock, that both insects and myriopods have descended from Scolopendrella, we do not agree, because this form has so many insectean features, and a single unpaired genital opening. For the same reason we should not agree with Schmidt in interpolating the Symphyla between the Pauropoda and Diplopoda. In these last two progoneate groups the genital openings are paired, hence they are much more primitive types than Scolopendrella, in which there is but a single opening. It seems most probable that the Symphyla, though progoneate, are more recent forms than the progoneate myriopods, which have retained the primitive feature of double sexual outlets. It is more probable that the Symphyla were the descendants of these polypodous forms. Certainly Scolopendrella is the only extant arthropod which, with the sole exception of the anteriorly situated genital opening, fulfils the conditions required of an ancestor of Thysanura, and through them of the winged insects. No one has been so bold as to suggest the derivation of insects from either diplopods or chilopods, while their origin from a form similar to Scolopendrella seems not improbable. Yet Uzel has very recently discovered that Campodea develops in some respects like Geophilus, the primitive band sinking in its middle into the yolk, with other features as in chilopods.[[7]] The retention of a double sexual opening in the diplopods is paralleled by the case of Limulus with its double or paired sexual outlets, opening in a pair of papillæ, as compared with what are regarded as the generalized or more primitive Crustacea, which have an unpaired sexual opening.