BRACHIOPODA of Megathyris and Terebratulina which immediately precede fixation. The cirri or tentacles, of which three or four pairs are present, are capable of being protruded, and the minute larva swims by means of the ciliary action they produce. It can retract the tentacles, shut its shell, and sink to the bottom. Beecher (Amer. Jour. Sci., loc. cit.) has classified with appropriate names stages Fig. 5.—Stages in the lixing and metamorphosis the , i various , . . , of Terebratulina. Highly magnified ; from Morse. tlirOUgn WillCn A, larva (neo-embryo) just come to rest; B, C, I), Tdrnphirmnrl c steges showing the turning forward of the second 1 ‘ or mantle segment; E, completion of this; F, pass. The last Stage, t&egmentT^ b 2, 3, the first, second, and ^ in wllich the folds of the second segment are already reflected over the first, he calls the Typembryo. Either before or just after turning, the mantle develops a larval shell termed the protegulum, and when this is completed the larva is termed the Phylembryo. By this time the eyes have disappeared, the four bundles of .chsette have dropped off, and the lophophor has begun to appear as an outgrowth of the dorsal mantle-lobe. The protegulum has been found in members of almost all the families of Brachiopod, and it is thought to occur throughout the group. It resembles the shell of the Cambrian genus Paterina, and the Phylembryo is frequently referred to as the Paterina stage. In some orders the Phylembryo is succeeded by an Obolella stage with a nearly circular outline, but this is not universal. The larva now assumes specific characters and is practically adult. Classification.—Beecher’s division of the Brachiopoda into four orders is based largely on the character of the aperture through which the stalk or pedicle leaves the shell. To appreciate his diagnoses it is necessary to understand certain terms, which unfortunately are not used in the same sense by all authors. The triangular pedicle-opening seen in Orthis, &c., has been named by Hall and Clarke the delthyrium. In some less primitive genera, e.g., Terebratula, that type of opening is found in the young stages only; later it becomes partly closed by two plates which grow out from the sides of the delthyrium. These plates are secreted by the ventral lobe of the mantle, and were named by von Buch in Fig. 6.—Shell of larval Brachiopod. Phylembryo 1834 the “ delti- stage, x 90; from Simroth. 1, protegulum ; 2, dium.” The form permanent shell. of the deltidium varies in different genera. The two plates may meet in the middle line, and leave only a small oval opening near the centre for the pedicle, as in Phynchonella; or they may meet only near the base of the delthyrium forming the lower boundary of the circular pedicle-opening, as in Terebratula; or the right plate may remain quite distinct from the left plate, as in Tereratella. The pro-deltidium, a term introduced by Hall
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and Clarke, signifies a small embryonic plate originating on the dorsal side of the body. It subsequently becomes attached to the ventral valve, and develops into the pseudo-deltidium, in the fteotremata and the Protremata. The pseudo-deltidium (so named by Brown in 1862) is a single plate which grows from the apex of the delthyrium downwards, and may completely close the aperture. The psendo-deltidium is sometimes reabsorbed in the adult. In the Telotremata neither prodeltidium nor pseudo-deltidium is known. In the Atremata the pro-deltidium does not become fixed to the ventral valve, and does not develop into a pseudodeltidium. The American use of the term deltidium for the structure which Europeans call the pseudo- Fig. 7.—Diagramof the pedicle-opening deltidium makes for con- fififinehonella 1, umbo . of ventral valve;Magnified. 2, deltidium
- 3,
fusion. I he development margin of delthyrium; 4, pedicleopening: 5 dorsal valve of the brachial supports has ’ ‘ been studied by Friele, Fischer, and Oehlert. A summary of the results is given by Beecher {Trans. Connect. Acad. ix., 1893). The orders Atremata and Neotremata are frequently grouped together, as the sub-class Inarticulata or Ecardines — the Tretenterata of Davidson — and the orders Protremata and Telotremata, as the Articulata or Testicardines—the Clistenterata of Davidson. The following scheme of classification is based on Beecher’s and Schuchert’s. Most of the families mentioned are described by Davidson. Recent families are printed in italic type. ORDER I. Atremata (Beecher). — Inarticulate Brachiopoda, with the pedicle passing out between the unibones, the opening being shared by both valves. Pro-deltidium attached to dorsal valve. FAMILIES.—Paterinida:, Oifoi.in.n, Trimerellida:, Lingulellidte, Lingulw.e, Ligulasmatida). ORDER II. Neotremata (Beecher).-—More or less circular, coneshaped inarticulate' Brachiopoda. The pedicle passes out at right angles to the plane of junction of the valves of the shell; the opening is confined to the ventral valve, and may take the form of a slit, or may be closed by the development of a special plate called the listrium, or by a pseudo-deltidium. Pro-deltidium attached to ventral valve. FAMILIES.-—Acrotretid.e, SiphonoTRETIDiE, TrEMATIDA, IJlSCINIDJi, GllANUlJsV. ORDER III. Protremata (Beecher).—Articulate Brachiopoda, with pedicle-opening restricted to ventral valve, and either open at the hinge line or more or less completely closed by a pseudo-deltidium, which may disappear in adult. The pro-deltidium originating on the dorsal surface later becomes anchylosed with the ventral valve. FAMILIES.—Kutorginida, Eichwaldiida, Billingsellida, Strophomenida, Tiiecidiid.e, Productida, RichthoFENIDA, OrTHIDA, C LIT AMBONITI HA, SVNTROPHIIDA, PORAMBONITIDA, PeNTAMERIDA. ORDER IV. Telotremata (Beecher).—Articulate Brachiopoda, with the pedicle-opening, confined in later life to the ventral valve, and placed at the umbo or beneath it. Deltidium present, but no pro-deltidium. Lophophor supported by calcareous loops, &c. FAMILIES.—Protorhynchida, Rhynchonellid-e, Centroneli.ida, Terebratulid.e, Stringocephalida, Megalanterida, Terebratellib.e, Atrypida, Spiriferida, Athyrida. A ffinities.—Little light has been thrown on the affinities of the Brachiopoda by recent research, though speculation has not been wanting. Brachiopods have been at various times placed with the Mollusca, the Chsetopoda, the Chsetognatha, the Phoronidea, the Polyzoa, the Hemichordata, and the Urochordata. Hone of these alliances have borne close scrutiny. The suggestion to place Brachiopods with the Polyzoa, Phoronis, Rhabdopleura, and Cephalodiscus in the Phylum Podaxonia made in volume xix. of the ninth edition (pp. 440-41) has not met with acceptance, and until we have a fuller account of the embryology of some one form, preferably an Inarticulate, it is wiser to regard the group as a very isolated one.
