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 enclo ses 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 inf ormation on its contractile and passive mechanical properties. The ECS form s five interambulacral pouches each of which has adthecal (test-facing) and adambulacral (ambulacrum-facing) walls. The ECS wall comprises two coeloth elia separated by a layer of connective tissue. The outer coelothelium is a single layer of monociliated cuboidal peritoneocytes and basally located a xon-like processes. The inner coelothelium is a single layer of squamous pe ritoneocytes overlying axon-like processes and, in the adthecal regions onl y, parallel arrays of elongated myocytes orientated obliquely or horizontal ly. The intraseptal connective tissue consists mainly of collagen fibrils w ith 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 contain s calcite spicules of variable size, shape, abundance and orientation. Isol ated preparations of the ECS show concentration-dependent contractile respo nses to K+ ions and acetylcholine. The magnitude of the contractile force v aries with the vertical position of the lantern (which determines the start ing length of the ECS) in an unusual pattern. Cyclical loading-unloading te sts indicate that, as the lantern is raised, the ECS shows low stiffness un til the lantern reaches its normal resting position. It is concluded that t he adthecal regions of the ECS help to set a limit to lantern retraction an d that their contractility assists the protractor muscles in exerting a dow nward pull on the lantern.