Locomotion and propagated skin impulses in salps (Tunicata: Thaliacea).

Abstract

1. Various observations by M. Fedele on the mechanism of forward and reverse locomotion, on the neurogenic origin of the locomotor rhythm and on the coordinated behavior of salp chains are confirmed or extended. Salpa fusiformis was the species chiefly studied. 2. The striated muscle fibers of the body wall exhibit nonpropagative, graded responsivity. The fibers are multiply-innervated. Adjacent fibers are not electrically coupled. 3. Intracellular recordings are reported from a pacemaker and presumed motor neurons in the brain. The locomotor rhythm is exhibited by deafferented and isolated brains. In the intact animal, sensory input can modify the rhythm and alter the firing sequence of the muscles. The rhythm is accelerated by reduction, and inhibited by elevation of the ambient light intensity. 4. The outer skin is a conducting epithelium. The cells conduct action potentials at ca. 17 cm/sec and are connected by gap junctions. Three other independently conducting inner epithelial territories are described. Propagated impulses in the excitable epithelia are believed to enter the nervous system via neurosensory processes in the skin, extending the effective fields of these receptors. 5. Salp chains show coordinated responses but, except in their earliest developmental stages, impulses are probably not through-conducted along the chain, but are relayed from one zooid to the next by an unknown mechanism. 6. Comparisons are drawn between salps and other pelagic tunicates where conducting epithelia have previously been reported.