A spicule-reinforced contractile mesentery: organisation and mechanical behaviour of the exterior coelomic septum of Stylocidaris affinis (Echinodermata, Echinoida)

The exterior coelomic septum (ECS) is a mesentery-like structure that encloses the lantern of regular sea-urchins and connects it to the inner surface of the test. This paper describes the ultrastructure and microarchitecture of the ECS in Stylocidaris affinis (Cidaridae, Echinoida) and provides information on its contractile and passive mechanical properties. The ECS forms five interambulacral pouches each of which has adthecal (test-facing) and adambulacral (ambulacrum-facing) walls. The ECS wall comprises two coelothelia separated by a layer of connective tissue. The outer coelothelium is a single layer of monociliated cuboidal peritoneocytes and basally located axon-like processes. The inner coelothelium is a single layer of squamous peritoneocytes overlying axon-like processes and, in the adthecal regions only, parallel arrays of elongated myocytes orientated obliquely or horizontally. The intraseptal connective tissue consists mainly of collagen fibrils with sparsely distributed spherule cells and cells containing heterogeneous vesicles. In the adambulacral regions of the ECS hollow beaded microfibrils 20-23 nm in diameter form fibre-like aggregations. This layer also contains calcite spicules of variable size, shape, abundance and orientation. Isolated preparations of the ECS show concentration-dependent contractile responses to K+ ions and acetylcholine. The magnitude of the contractile force varies with the vertical position of the lantern (which determines the starting length of the ECS) in an unusual pattern. Cyclical loading-unloading tests indicate that, as the lantern is raised, the ECS shows low stiffness until the lantern reaches its normal resting position. It is concluded that the adthecal regions of the ECS help to set a limit to lantern retraction and that their contractility assists the protractor muscles in exerting a downward pull on the lantern.