Reextension mechanism of a scyphistoma's tentacle

Abstract

The tentacles of Aurelia's scyphistoma are composed of ectodermal epitheliomuscular cells, a thin mesoglea, and an endoderm consisting of a single row of vacuolated cells lacking circular myofibrils. Tentacular shortening results from a shortening of the sheath-like collection of longitudinally disposed ectodermal myofibrils but reextension is effected by viscoelastic forces most likely residing in the mesoglea whose fibrils dissolve in collagenase as shown by the electron microscope. As the tentacle shortens it thickens and apical expansion folds open out; also, the cylindrical endodermal cells shorten, bulge, and pull in a wedge of highly strained mesoglea at the margin where neighboring core cells meet. The rate of tentacular reextension can be measured and analyzed. After brief periods of stimulation which contract the tentacle for a short time, reextension is fast at first, then slows later: after progressively longer periods in the shortened state there is a concomitant decrease in the fast phase and increase in the duration of the slow phase of reextension. The reextending force is best represented by a spectrum of retardation times, and like typical viscoelastic materials the distribution of retardation times is constant over relatively long times.