=The Arachnida.=—The scorpions and spiders are much less closely allied to the myriopods and insects than formerly supposed. Their embryology shows that they have descended from forms related to Limulus, possibly having had an origin in common with that animal, or having, as some authors claim, directly diverged from some primitive eurypteroid merostome. But they differ in essential respects, and not only in the nature and grouping of their appendages; the first pair instead of antenniform being like mandibles, and the second pair like the maxillæ, with the palps, of insects, the four succeeding segments (thoracic) bearing each a pair of legs. They also have a brain quite unlike that of Limulus, the nervous cord behind the brain, however, being somewhat similar, though that of Limulus differs in being enveloped by an arterial coat. Arachnida respire by tracheæ, besides book-lungs, which, however, are possibly derivatives of the book-gills of Limulus, while they perform the office of excretion by means of the malpighian tubes, and like Limulus possess two large digestive glands (“liver”). Their embryos have, on at least six abdominal segments, rudiments of limbs, three pairs of which form the spinnerets, showing their origin from Limulus-like or eurypteroid forms; their coxal glands are retained from their eurypteroid ancestors. The Arachnida probably descended from marine merostomes, and not from an independent annelid ancestry, hence we have represented them in the diagram on p. 3 as branching off from the merostomatous phylum, rather than from an independent one. [Illustration: 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.