This paper provides the first detailed account of the histological and ultrastructural organisation of an echinoderm ligament that is non-mutable. Each of the five compass ossicles on the aboral side of the lantern of regular echinoids is linked to an underlying rotular ossicle by a compass-rotular ligament (CRL). The structure and anatomical relations of the CRL of Echinus esculentus L. were examined by light microscopy and scanning and transmission electron microscopy, and its responsiveness to neuroactive agents was observed in stress relaxation tests. The CRL consists of (1) thick collagen fibres that form an outer crossed fibre lattice and an internal system of parallel suspensory fibres and (2) regions composed mainly of bundles of ca. 12 nm microfibrils. The CRL is sparsely cellular, most cell bodies belonging to heterogeneous vesicle-containing cells that undergo apoptosis-like cytoplasmic fragmentation and produce linear aggregations of cell fragments. Coelothelia investing the CRL show intense secretory activity reminiscent of lamellar body production by mammalian mesothelia and are the source of bundles of cell processes that penetrate deeply into the CRL, branch sparingly and terminate between the thick fibres. Isolated CRL preparations are unresponsive to seawater containing 1 mM acetylcholine or 100 mM K + , but show a transient increase in stress relaxation rate when treated with 0.56 M KCl. The effects of solutions of other alkali metal chlorides suggest that the latter response is due to the direct action of KCl on extracellular components of the CRL and is not nervously mediated.

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