Quarterly Journal of Microscopical Science, Vol s3-101, 339-350, Copyright © 1960 by Company of Biologists

Contractility and Hydroplasmic Movements in the Hydroid Clytia Johnstoni

¹ Department of Zoology, University of Edinburgh

The investigation is based on observations of the living animal and on time-lapse films, and is supplemented by histological preparations. The material was cultured in the laboratory.

A short region just behind the growing tip of a stolon is contractile, pulsating every 4 or 5 min but occasionally at longer intervals. This causes the hydroplasm (contents of coelenteron) to ebb and flow in the coelenteron not only directly, but also by pressure transmitted by the fluid between the coenosarc and perisarc. There is evidence that where a stolon has more than one contractile region, they beat independently. The contractile cells do not possess muscle-tails, and contractile fibres have not been detected by light microscopy.

The hydranth stomach also has slow contractions which are peristaltic towards the stalk. They usually occur at irregular intervals but are occasionally regular. They cause movement of the hydroplasm into and out of the hydranth. There is evidence that hydranths are stimulated to expand and contract by arrival of hydroplasm pressed there by another contractile region of the colony. Longitudinal muscle-tails are present on the ectoderm cells of the stomach and tentacles, but these are probably concerned with retraction of the hydranth into its hydrotheca.

Although the endodermal flagella beat actively when the hydroplasm is in motion, they are not the cause of hydroplasmic flow.

The possible mechanism of these slow contractions is discussed in relation to contractility in other coelenterates.