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The Arachnida.

Fig. 3.—Paired genital openings of different classes of arthropods. A, the most primitive, of Limulus polyphemus: gen. p, generative papillæ; d, duct; vd, vas deferens; t, tendinous stigmata; stig, stigmata; e, external branchial muscle; ant, anterior lamellar muscle.—After Benham, with a few changes. B, lobster (Homarus vulgaris), ♀: oe, genital aperture on 3d pair of legs; ov, ovary; u, unpaired portion of the same; od, oviduct. C, ♀, scorpion: ov, ovary, with a single external opening. D, ♂: t, testis; vd, vasa deferentia; sb, seminal vesicle; a, glandular appendage; p, penis.—After Blanchard. E, a myriopod (Glomeris marginata, ♀): os, ovarian sac, laid open; od, paired oviducts. F, ♂: t, testis; gvd, common vas deferens; pa, paired ducts.—After Favre, from Lang. G, Lepisma saccharina, young ♂: vd, vas deferens, ed, ejaculatory duct; ga, external appendages.—After Nassonow. H, Ephemera, ♂, showing the double outlets.—After Palmén.

The characters in which arachnids approach insects, such as tracheæ and malpighian tubes (none occur, as a rule, in marine or branchiate arthropods), may be comparatively recent structures acquired during a change from a marine to a terrestrial life, and not primitive heirlooms.