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Arachnida also show their later origin than merostomes by the fact that their sexual glands are in most cases single, and though with rare exceptions the ducts are paired, these finally unite and open externally by a common single genital aperture in the median line of the body, at the base of the abdomen (Fig. 3, C, D). In this respect Limulus, with its pair of genital male or female openings, situated each at the end of a papilla, placed widely apart at the base of the first abdominal limbs, is decidedly more archaic. Unlike Crustacea and insects, Arachnida do not, except in the mites (Acarina), which is a very much modified group, undergo a metamorphosis.

We see, then, that the insects, with the Myriopoda, are somewhat isolated from the other Arthropoda. The Myriopoda have a single pair of antennæ, and as they have other characters in common with insects, Lang has united the two groups in a single class Antennata; but, as we shall see, this seems somewhat premature and unnecessary. Yet the two groups have perhaps had a common parentage, and may prove to belong to a distinct, common phylum.