Online Encyclopedia


Online Encyclopedia
Originally appearing in Volume V04, Page 366 of the 1911 Encyclopedia Britannica.
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TEREBRATULIDAE NELLIDAE,STRINGOCEPHALIDAE, MEGALANTERIDAE, TEREBRATELLIDAE, ATRYPIDAE, SPIRIFERIDAE, ATHY RIDAE. Affenities.—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 Chaetopoda, the Chaetognatha, the Phoronidea, the Polyzoa, the Hemichordata, and the Urochordata. None of these alliances has borne close scrutiny. The suggestion to place Brachiopods with the Polyzoa, Phoronis, Rhabdopleura and Cephalodiscus, Iii the Phylum Podaxonia made in Ency. Brit. (vol. xix, ninth edition, pp. 440-441) has not met with acceptance, and until we have a fuller account of the embryology of some one form, preferably an In-articulate, it is wiser to regard the group as a very isolated one. It may, however, be pointed out that Brachiopods seem to belong to that class of animal which commences life as a larva with three segments, and that tri-segmented larvae have been found now in several of the larger groups. Distribution.—Brachiopods first appear in the Lower Cambrian, and reached their highest development in the Silurian, from which upwards of 2000 species are known, and were nearly as numerous in the Devonian period ; at present they are represented by some 140 recent species. The following have been found in the British area, as defined by A. M. Norman, Terebratulina caput-serpentis L., Terebratula (Gwynia) capsula Jeff., Magellania (Macandrevia) cranium Miill.,M. septigera Loven,Terebratella spitzbergenensis Day., Megathyris decollata Chemn., Cistella cistellula S. Wood, Cryptopora gnomon Jeff., Rhynchonella (Hemithyris) psittacea Gmel., Crania anomala Mull., and Discinisca atlantica King. About one-half the 120 existing species are found above the too-fathoms line. Below 150 fathoms they are rare, but a few such as Terebratulina wyvillei are found down to 2000 fathoms. Lingula is essentially a very shallow water form. As a rule the genera of the northern hemisphere differ from those of the southern. A large number of specimens of a species are usually found together, since their only mode of spreading is during the ciliated larval stage, which althoughyit swims vigorously can only cover a few millimetres an hour; still it may be carried some little distance by currents. Undue stress is often laid on the fact that Lingula has come down to us apparently unchanged since Cambrian times, whilst Crania, and forms very closely resembling Discina and Rhynchonella,, are found from the Ordovician strata onwards. The former statement is, however, true of animals from other classes at least as highly organized as Brachiopods, e.g. the Gasteropod Capulus, whilst most of the invertebrate classes were represented in the Ordovician by forms which do net differ from their existing representatives in any important respect. A full bibliography of Brachiopoda (recent and fossil) is to be found in Davidson's Monograph of British Fossil Brachiopods, Pal. Soc. Mon. vi., 1886. The Monograph on Recent Brachiopoda, by the same author, Tr. Linn. Soc. London, Zool. ser. ii. vol. iv., 1886-1888, must on no account be omitted. (A. E. S.)

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