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The following summary of the structural features of the Symphyla, as represented by Scolopendrella, is based mainly on the works of Grassi, Haase, and Schmidt, with observations of my own.

Diagnostic or essential characters of Symphyla.

Trunk with from fifteen to sixteen dorsal, more or less free subequal scutes, the first the smallest. Pedigerous segments twelve; also twelve pairs of 5–jointed legs, which are of nearly equal length, the first pair 4–, the others 5–jointed, all ending in two claws, as in Synaptera and winged insects. A pair of 1–jointed anal cerci homologous with those of Thysanura and Orthoptera, into each of which opens a large abdominal silk-gland. Abdominal segments with movable styles or “pseudopods” (“Parapodia” of Latzel and of Schmidt), like those of Campodea and Machilis, and situated on the base of the coxal joint in front of the ventral sac. Within the body near the base of each abdominal style is an eversible coxal sac or blood-gill (Fig. 15, cg). The single genital opening is on the fourth trunk-segment in both sexes (Fig. 15, indicated by the arrow). The malpighian tubes (ur. t) are two in number, opening into the digestive canal at the anterior end of the hind intestine; they extend in front to the third or second segment from the head. They are broad and straight at their origin, becoming towards the end very slender and convoluted.