Muscle system organization in the Echinoderms. I. Intervertebral muscles of Ophioderma longicaudum (Ophiuroidea)

The intervertebral muscles of the arm of the serpent star Ophioderma longicaudum have been observed under the electron microscope. These fibers, enabling the arm to move in all directions, show an ultrastructural morphology not easily ascribed to any muscle fiber category. They are internally organized into scaled bundles of filaments, determining, in cross section, alternating A- and I- zones. Inside these bundles the homologous filaments are staggered lengthwise and distributed according to a regular pattern. The arrangement into step-like bundles is emphasized by the distribution of the Z-elements, regularly inserted in the middle of the I-zone. These muscles, therefore, have an organization neither recalling that of a cross striated muscle nor that of a smooth muscle. More possibilities of comparison can be made with the helical muscle systems. The most evident analogies may be found in the myofilament arrangement of some molusc muscles (abductor muscle of the oyster), both in the distribution of the contractile elements and in the probable paramyosinic axis inside the thick filaments. The muscles of the Ophioderma arm, however, if compared with the 'classic helical muscle', lack a precise preferential obliquity, defined by the stagger of the filaments and of the Z-bodies toward a unique direction. These muscles, therefore, probably represent a model of fiber verging on the helical organization.