Morphogenetic Events Associated with Stolon Elongation in Colonial Hydroids

Abstract

The growth of stolons anil the development of hydranths in thecate hydroids (Cnidaria, Hydrozoa) have several common features. In both the terminal zone shows rhythmic pulsations which accompany periodic advances and withdrawals of the tip. Behind the terminal zone is a longer region of coenosarc which shows vigorous contractions, and these produce flow of the contents of the gastrovascular cavity. These contractions are not directly related to the movement of the tips of pedicels and stolons, nor are they related to contractions within a developing hydranth. The growing regions are not pushed out from behind, but figuratively pull more proximal tissue along. The changes in shape must involve active changes within the cells. The terminal cells are not “typical” myoepithelial cells (with “muscle tails”). How they alter their shape is not yet known. Evidence from dissociation studies and from treatment with drugs indicates a critical stage in hydranth development before which tissues are indeterminate, after which a mosaic of the hydranth is established.