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Evolutionary Patterns of Axial Muscle Systems in Some Invertebrates and Fish

Q. Bone
American Zoologist
Vol. 29, No. 1 (1989), pp. 5-18
Published by: Oxford University Press
Stable URL: http://www.jstor.org/stable/3883329
Page Count: 14
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Evolutionary Patterns of Axial Muscle Systems in Some Invertebrates and Fish
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Abstract

Axial muscles used for oscillatory swimming are found not only in fish and other vertebrates but also in some protochordates and invertebrates. Chaetognaths have unsegmented locomotor musculature with some unusual features, but larvacean tunicates and the tadpole larvae of ascidians show the simplest variant of the chordate segmented axial muscle arrangement for flexing a notochordal column, where all muscle cells along one side are electrically coupled. With amphioxus, the basic fish myotomal layout is established, with two main fibre types probably used for different patterns of swimming (as in fish). There are, however, several unique features, including the flattened fibre shape and the paramyosin system of the notochord. Agnatha have two fibre types in the myotomes, a third type perhaps being a developmental stage in the ontogeny of fast fibres. In lampreys, the central fibres of the characteristic fibre sandwiches in the myotomes are flattened (though less so than in amphioxus); they have a dual innervation of unknown function seen also in the fast fibre system of many Gnathostome fish groups. Hagfish fast fibres are not flattened nor do they have a dual innervation. Gnathostome fish axial muscles are strikingly uniform in design with two possible exceptions: (1) higher teleost fast fibres which, unlike those of other groups, are multiply-innervated and (2) tonic fibres in a few fish, which seem not to be involved in locomotion.

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