ABSTRACT
Cerianthus membranaceus, as commonly available at Naples, is a brownish polyp which can extend to 15–20 cm. and contract to 5–7 cm. The numerous tentacles are less contractile, being usually 3-5 cm. long. The gross anatomical structure is well described by Hyman (1940). The polyp normally lives in a soft tube, which it secretes, and has no foot or sphincter muscle. Lacking the diversity of muscle groups of a typical sea anemone its movements are very restricted. In contrast to other coelenterate polyps the Ceriantharia have a substantial ectodermal musculature of the column, which is responsible for the vigorous withdrawal of the animal into its tube. There are also ectodermal radial fibres on the disk. Endodermal muscles are relatively insignificant and play no part in the responses considered here. In common with all other coelenterate polyps Cerianthus has two kinds of rapid response to external changes: the feeding movements and the protective retraction. In Cerianthus the two are aroused by different stimuli, and in part they are effected by different muscles. The present study is an analysis of some of the physiological properties of the pathways which co-ordinate these responses.
