The contractile system of hydra, as studied with saponin-extracted models and electron microscopy

Motile systems have been categorized by Hoffmann-Berling (1960) and H.H. Weber (1958) on the basis of the action of ATP on various types of movement. The means by which ATP manifests its effect on motility is usually, but not always, by stimulation of contraction. Although this is the nature of its action on movement of most cells, including higher muscle·fibers (see Szent-Gyorgyi, 1953), ameba (see Hoffmann-Berling, 1960; Simard-Duquesne and Couillard, 1962), and dividing fibro-blast cells (Hoffmann-Berling, 1953, 1954; Hoffmann-Berling and H.H. Weber, 1953), there exist systems in which ATP stimulates elongation (e.g., the spindle during mitosis; Hoffmann-Berling amd Portzehl, 1955). Still other movements are inhibited or reversed by ATP, including the contraction of vorticella. stalks (Hoffmann-Berling, 1958) and the elongation
of trichocysts of certain ciliates (see Hoffmann-Berling, 1960; H.H. Weber, 1958). Finally, rhythmic movements of flagella and cilia are activated by ATP (Hoffmann-Berling. 1954, 1959, 1960); here it cannot be determined whether ATP is activating the contractile or the relaxing phase of the movement.

Contraction in hydra, a member of the most primitive group
of multicellular organisms (the Coelenterates) capable of coordinated
movement, is the subject of this investigation. Hydra bears functional similarities in its rapid, nerve conducted epidermal shortening to vertebrate striated, skeletal muscle, whereas its ;slow, peristaltic gastrodermal elongation is functionally similar to movement of smooth, visceral muscles (Mueller, 1950). On the other hand, the periodic, spontaneous contractions of hydra superficially resemble those of vorticella (Jennings, 1906, p. 189 to 190).