Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 84 BRACHIOPODA. of which the anterior becomes umbrella-shaped, and develops cilia and eye-spots; subsequently it atrophies and gives rise to the upper lip. A fold is formed on the middle segment; this gives rise to the two mantle lobes, which soon cover the body and a part of the caudal segment (fig. 556, d). Four bundles of long setse, which, as in the Worms, can be drawn in and protruded, make their appearance on the ventral lobe of the mantle of the developing larva. Later the larva b
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 84 BRACHIOPODA. of which the anterior becomes umbrella-shaped, and develops cilia and eye-spots; subsequently it atrophies and gives rise to the upper lip. A fold is formed on the middle segment; this gives rise to the two mantle lobes, which soon cover the body and a part of the caudal segment (fig. 556, d). Four bundles of long setse, which, as in the Worms, can be drawn in and protruded, make their appearance on the ventral lobe of the mantle of the developing larva. Later the larva becomes attached and the metamorphosis begins. The fixed posterior segment becomes the peduncle; the mantle lobes bend forward and produce the shell. The bundles of setae are thrown off; the deposition of calcareous matter in the shell begins, and the tentacular filaments (which are at first arranged in a circle) of the later arms make their appearance. In Thecidium the inner layer (mesoderm and endoderm) arises from masses of cells which are budded off into the segmentation cavity. The subsequent meta- morphosis of the larva when provided with tentacles has been most accurately investigated by Brooks for Linyula, the larvae of which are still free-swimming when the tentacles are being developed (fig. 557, a, b). At the present day but few Brachiopods are found in the different seas, as compared with the much larger number in the earlier formations; certain species of these fossil Brachiopods have great importance as characteristic fossils. The oldest fossils also belong to the Brachiopoda and certain genera which first appeared in the Silurian have persisted to the present day (Lingitla). M- FIG. 557 6.—Longitudi- nal section of an older larva (after Brooks). Do, Dorsal; Ve, ven- tral valve of the shell; Mr, thickened mantle edge; T, tentacles ; 0, mouth ; Md, stomach ; Ad, intestine; M, pos- terior muscle; G, gang- lion. Order 1.—ECARDINES (INARTICULATA.). Shell without
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