ABSTRACT
Ultrastructure of imperforate stereom in echinoderm ossicles ABSTRACT: The ultrastructure of two kinds of ossicles provided with imperforate stereos is described using SEM. The glassy bodies of Leiaster leachi (Asteroidea) consist of compact bulges stemming from the layer of perforate stereos that covers the outer surface of the skeletal plate. The glassy bodies of Stellaster equestris (Asteroidea) are discrete compact structures composed of a root and a lens-like structure that protrudes from the outer surface of the ossicle. The sphaeridia of Psammechinus miliaris and Echinocardium cordatum (Echinoidea) are composed of two distinct skeletal regions, viz. the stalk (which is made of labyrinthic stereom) and the globule (which is principally made of imperforate stereom). Imperforate stereos of both glassy bodies of S.equestris and sphaeridiun globules of P.miliaris and E.cordatum probably results from a secondary thickening of an initial low density stereom, which leads to the occlusion of the stereos pores. This hypothesis probably applies to the formation of all optically monocrystalline dense stereoms of echinoderms.
