Muscles à striation oblique et à striation transversale chez les Annélides Polychètes

Abstract

Ohlique- and cross-striated muscles in polichaetes.

Most annelids, chiefly polychaetes, have muscles of oblique double striation or of helical striation. Yet special evolutionary processes may induce profound changes in the structure of muscles in two polychaete families, the Syllidae and Nereidae. At time of reproduction they can undergo somatic metamorphoses termed stolonisation and epitoky. Many of their muscles de-differentiate partly or wholly, and then they redifferentiate into epitokous fibres, characterized by thick, shorter filaments of smaller diameter, and rich in mitochondria and glycogen. Besides, there are some examples of muscle fibres non-helical in type worth investigating, for they show, first, an absence of a direct relation between phylogeny and fibre type (obliquely or transversely striated, or smooth), and, secondly, an absence of transitions between smooth and cross-striated fibres (the proventriculus in Syllidae), and between the latter and fibres of oblique simple striation (Magelona). Concerning the cross-striated muscle fibres of Magelona, several kinds are described. They seem of more primitive structure than those of some lower groups, such as the cnidarians and the flatworms (tail muscles of the cercaria), but they can be likened often to certain fibres found in arthropods; moreover, their contractions are apparently slower than in other cross-striated fibres. The presence of such fibres in an annelid raises the question of their relationship to a hydrostatic skeleton.