Abstract
The major tissues of a horsehair worm (Gordius sp., Nematomorpha) were studied by transmission and scanning electron microscopy. The epicuticle is a thin superficial layer whose surface shows rows of polygonal elevations. The cuticle has many layers of cylindrical, nonperiodic collagenous fibers, spirally wound about the long axis of the worm. Alternating layers of fibers cross one another diagonally at angles of approximately 60° and 120°. Additionally there are large radial fibers and strata of small fibers. The epidermis is a single layer of highly interdigitated cells. The musculature consists of overlapping, longitudinally arranged, flat cells, each containing many thick and thin filaments in anteroposterior orientation. Circular muscles are absent. Striated collagenous fibers completely surround the following: the muscle cells, a degenerate gut, a nerve cord composed of neural and glial cells, and mesenchyme that nearly fills the pseudocoel. Functional, developmental, and comparative morphology is discussed.
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