. Natural history. Zoology. LAMPSFIELLS, ETC. 649 long muscular stalk, called the peduncle fFig. 2). Tliis larger valve is there- fore often called the "' peduncle valve " ; by English writers it is called the " ventral valve," although in the natural position of stalked forms it is always the uppermost. The smaller valve is called the "brachial" or "dorsal" valve. In microscopic structure, also, the shell differs from that of the Mollusoa (Fig. 3). The two valves may be united by a hinge at the hinder, or umbonal end (Articulata, Fig. 4), or thoy may no
. Natural history. Zoology. LAMPSFIELLS, ETC. 649 long muscular stalk, called the peduncle fFig. 2). Tliis larger valve is there- fore often called the "' peduncle valve " ; by English writers it is called the " ventral valve," although in the natural position of stalked forms it is always the uppermost. The smaller valve is called the "brachial" or "dorsal" valve. In microscopic structure, also, the shell differs from that of the Mollusoa (Fig. 3). The two valves may be united by a hinge at the hinder, or umbonal end (Articulata, Fig. 4), or thoy may not (Inarticulata, Fig. 5). The shell-valves are lined by two mantle-folds, or extensions of the body- walls, which contain prolongations of the body-cavity ; and in them the generative products are formed. The free margins of the mantle-folds are beset with bristles (setre, shown at 6. in Fig. 2). Muscles pass across the body of the animal from one valve to the other; tliey serve to open and to close the valves, and to move them sideways. The imprints of these muscles on the shell are important in the study of fossil braohiopods (Fig. 5).. Fif/. 4.âA fTiNiJEU Brachiopod (2;a'ines(j[m7ia a^^frnaifl). * The upper figure shows the hinge from outside; the lower figures show the inside of the valves, , peduncular or ventral valve; !).b., brachial or dorsal valve ; ^., hinge-line ; d., deltidium ; /., foramen ; a., area; and , its inner and outer portions ; 1, tooth; , socket in which tooth worlts; , car- dinal process; , muscle scars. Two-thirds natural size. The' viscera (Fig. 6) lie near the hinder umbonal part of the shell, and the â mouth is directed towards the fore-part or opening of the shell. Around the mouth is a somewhat horseshoe-shaped disc, bearing ciliated tentacles, and called the lophophore (tuft-bearer). It is often produced into two processes or "arms," which fill the fore-part and sides of the shell-cavity, and are often spirall
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Keywords: ., bookauthorly, bookcentury1800, bookdecade1890, booksubjectzoology
