Fig. 1. Polyorchis morphology. (A) Longitudinal section through the centre of the animal. (B) Cross-section taken at the location of the double-headed arrows in A. (C) Enlargement of one quadrant of B. The mesoglea is divided into two regions [labelled BM (bell mesoglea) and JM (joint mesoglea)] by the gastrodermal lamella (GL). During contraction, since the circumferential swimming muscle (CM) is only attached to the mesoglea at the per- [PR; at radial canals (RC)] and inter-radii (IR; away from RC), the bell first folds around the adradial joints (AR), and only later in the contraction begins to stretch the radial fibres (RF). Dimensions indicated are the bell height (h) from the apex (A) to the margin; shoulder height (h_s) between the shoulder and the margin; the radius at the base (r₀) and the wall thickness (). Other structures of interest: E, exumbrellar epithelium; S, subumbrellar epithelium; P, peduncle; M, manubrium; V, velum; RM, radial muscle. Drawings modified from Gladfelter (1972).