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Conducting epithelia are known to play an important part in the behavior of several groups of animals including hydrozoan coelenterates and amphibian larvae (reviewed by Mackie, 1970;Spencer, 1974), and skin conduction has recently been reported in pelagic tunicates and ascidian tadpoles (Bone and Mackie, 1975;Mackie and Bone, 1976).

Ctenophores are transparent marine organisms that swim by means of beating cilia; they are the simplest animals with individual muscle fibres. Predatory species, such as Beroe ovata, have particularly well-developed muscles and are capable of an elaborate feeding response. When Beroe contacts its prey, the mouth opens, the body shortens, the pharynx expands, the prey is engulfed and the lips then close tightly. How this sequence, which lasts 1 s, is accomplished is unclear. The muscles concerned are structurally uniform and are innervated at each end by a neuronal nerve net with no centre for coordination. Isolated muscle cells studied under voltage-clamp provide a solution to this puzzle. We find that different groups of muscle cells have different time-dependent membrane currents. Because muscle contraction depends upon calcium entry during each action potential, these different currents produce different patterns of contraction. We conclude that in a simple animal such as a ctenophore, a sophisticated set of membrane conductances can compensate for the absence of an elaborate system of effectors.

The giant smooth muscle fibers of a ctenophore were isolated by enzymatic digestion. These fibers are multinucleated cells, up to 50 ,um in diameter and 2 cm in length. Their ultrastructure and membrane electrical properties are similar to those of in situ fibers. Relaxed, coiled (partially contracted), and fully shortened states were distinguished in isolated cells and studied by scanning and transmission electron microscopy. Calciumcontaining mitochondrial granules were found in the coiled cells but not in either the relaxed or the fully shortened cells. The relaxed cell is characterized in cross section by the density of myosin filaments (457 ± 15 per jpm2) and the thin-to-thick filament ratio (5.2 ± 0.2). In the coiled cell, the muscle lattice does not expand uniformly, as shown by the variability of myosin spacing, and the thin-to-thick filament ratio decreases. Both clockwise and counterclockwise coiling occur along the same fiber. The implications of these findings with respect to the structure of the contractile apparatus are discussed.Giant smooth muscle fibers up to 50 Am in diameter and from 0.5 mm to several cm in length occur in the marine planktonic invertebrate Beroe ovata. Their ultrastructural features and basic electrophysiological properties have been described (1,2). However, the anatomical organization ofBeroe has hindered our understanding of these muscle cells when studied in situ. An unequivocal characterization ofthe contracted muscle at the ultrastructural level has yet to be done, and the electrophysiological study could not take full advantage of the extreme length of the longitudinal muscles due to complex overlapping of the cells. In addition, the biophysical properties of the intercellular matrix may interfere either by limiting the diffusion of various substances or by restricting the access of electrodes to deeply situated fibers. These latter difficulties were partly overcome by incomplete enzymatic digestion of the mesoglea (1).A new insight into the properties ofsmooth muscle has arisen from the use of cells isolated from various vertebrates (3-5). In this report, we describe a simple technique for isolating the giant muscle fibers of Beroe in viable form and for processing them for ultrastructural studies. A description of the isolated cell in relaxed, coiled, and fully shortened states is given. The comparison of these states leads us to propose a possible arrangement of the myofilaments in this particular muscle cell. MATERIAL AND METHODSSpecimens of Beroe ovata were collected at the Station Zoologique, Villefranche-sur-Mer, France. Cell Preparation. The procedure adopted takes advantage ofthe resealing properties ofthe muscle fiber ofBeroe (1). Thin slices of the body wall (2-4 cm long) from any level of the organism were dissected out. The length of the strip was parallel to the longitudinal axis of the animal so as to include intact longitudinal muscle fibers. The tissues were kept for 15 min in cooled artificial sea water (ASW; 500 mM NaCl/10 mM KCl/ 58 mM M...

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