Online Encyclopedia

NEMERTINA, or NEMERTEANS (Nemerlea)

Online Encyclopedia
Originally appearing in Volume V19, Page 364 of the 1911 Encyclopedia Britannica.
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NEMERTINA, or NEMERTEANS (Nemerlea), a subdivision of worms,' characterized by the ciliation of the skin, the presence of a retractile proboscis, the simple arrangement of the generative apparatus, and in certain cases by a peculiar pelagic larval stage to which the name " pilidium " has been given. Many of them are long thread-shaped or ribbon-shaped animals, more or less cylindrical in transverse section. Even the comparatively shortest species and genera can always be termed elongate, the broadest and shortest of all being the parasitic Malacobdella and the pelagic Pelagonemertes. There are no exterior appendages of any kind. The colours are often very bright and varied. Nemertines live in the sea, some being common amongst the corals and algae, others hiding in the muddy or sandy bottom, and secreting gelatinous tubes which ensheath the body along its whole length. Formerly, they were generally arranged amongst the Platyelminthes as a sub-order in the order of the Turbellarians, but with the advance of our know-ledge of these lower worms it has been found desirable to separate them from the Turbellarians and to look upon the Nemertina as a separate phylum. O. Burger classifies Nemertines into four orders: I. Protonemertini, in which there are two layers of dermal muscles, external circular and internal longitudinal; the nervous system lies external to the circular muscles; the mouth lies behind the level of the brain; the proboscis has no stylet; there is no caecum to the intestine. Families, CARINELLIDAE, HUBRECHTIIDAE. II. Mesonemertini, in which the nervous system has passed into the dermal muscles and lies amongst them; other characters as in Protonemertini. Family, CEPHALOTHRICIDAE. This order represents the Hoplonemertini of Hubrecht. IV. Heteronemertini, in which the dermal musculature is in three layers, an external longitudinal, a middle circular, an internal longitudinal; the nervous system lies between the first and second of these layers; the outer layer of longitudinal muscles is a new development; there is no intestinal caecum; no stylets on the proboscis and the mouth is behind the level of the brain. Families, EUPOLIIDAE, LINEIDAE. i Nemertes was a sea nymph, daughter of Nereus and Doris. One of the genera was named Nemertes by Cuvier. The Nematomorpha are nearly solid,—quite so at each end, From Cambridge Natural History, vol. ii., "Worms,"&c., by permission of Macmillan & Co., Ltd. 2 This order represents the Schizonemertini of Hubrecht and the family Eupolidae. The first three orders, which have a double muscular layer, external circular and internal longitudinal, are sometimes grouped together as the DIMYARIA; the Heteronemertini, in which a third coat of longitudinal muscles arises outside the circular layer, are then placed in a second branch, the TRIMYARIA. The following families and genera are represented on the British coasts: CARINELLIDAE, Cannella; CEPHALOTHRICIDAE, Cephalothrix, Carinoma; EJNEMERTIDAE, Eunemertes; OTOTYPHLONEMERTIDAE, Ototyphlonemertes; AMPHIPORIDAE, Amphiporus, Drepanophorus; TETRASTEMMIDAE, Tetra stemma, Prosorhocmus; MALACOBDELLIDAE, Malacobdella; EUPOLIIDAE, Eupo ia, Valencinia, Oxypolia; LINEIDAE, Lineus, Euborlasia, Micrura, Cerebratulus, Micrella.
End of Article: NEMERTINA, or NEMERTEANS (Nemerlea)
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