Text-book of comparative anatomy . ehind those already developed. Fre-quently, however, the segment bearing the chelicerse,and sometimes that bearing the pedipalps, appear onlyafter the formation of a few of the subsequent segments. 4. The rudiments of the extremities seem in variousArachnoidea to have different orders of succession. Thepermanent extremities, with the exception of the cheli-ceraa, which begin to form later, often develop simul-taneously. In the Psciulosforpionidce the rudiments ofthe extremities are even said to be recognisable beforethe marking off of the segments on the embr
Text-book of comparative anatomy . ehind those already developed. Fre-quently, however, the segment bearing the chelicerse,and sometimes that bearing the pedipalps, appear onlyafter the formation of a few of the subsequent segments. 4. The rudiments of the extremities seem in variousArachnoidea to have different orders of succession. Thepermanent extremities, with the exception of the cheli-ceraa, which begin to form later, often develop simul-taneously. In the Psciulosforpionidce the rudiments ofthe extremities are even said to be recognisable beforethe marking off of the segments on the embryonicrudiments. 5. In all Arachnoidea, except the lAnguatulidcc, thebody is, in its embryonic condition, more richly seg-mented than in the adult animal. The cephalo-thoracic region especially shows embryonic region consists at certain embryonic stages of acephalic or frontal lobe, in which the stomodfeum andthe definitive oral aperture form, and of 6 subsequentand thus post-oral segments, the 1st being that of the. -aba. FIG. 379.—Embryo of a Scor-pion, spread out flat, from theventral side (after Metschni-koff). kl, Frontal lobes; 1,chelicerse; 2, pedipalps; 3-6,the 4 pairs of legs; alia, rudi-ments of the abdominal limbs ;pa, post abdomen. chelicera?, the 2d that of the pedipalps, while the 4 others are the segments of the4 following pairs of extremities. In the abdomen also, even when there is no meta-merism in the adult animal, segmentation is to be recognised in the embryo. Thenumber of the embryonic abdominal segments in the various divisions of the Arach-noidea, however, differs greatly. It is a specially important fact that the segment bearing the chelicerse isin the embryo post-oral. No extremities develop on the frontal lobe, where, in theCrustacea, Protracheata, and Antennata, the antennae From this fact it followsthat there is no correspondence between the chelicerte of the Arachnoidea and theantennae of the Antennata. The appearance of rud
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
