Fine structure of the stalk of the bourgueticrinid sea lily Democrinus conifer (Echinodermata: Crinoidea)

Abstract

Specimens of Democrinus conifer (A. H. Clark, 1909) dredged from about 280 m were fixed aboard ship for transmission electron microscopy. The most important new contributions to stalk histology are the following: an exact description of the different cell types of the stereom spaces; a demonstration of the haemal channel; description of the radial aggregations of cells in the central canal; discovery of coelomic nerves; and discovery of nerve tracts running in association with the aboral extension of the axial organ. The collagenous ligaments of the stalk are separated into three types anatomically (and possibly also functionally). The stalk contains no trace of an axial sinus derived during ontogeny from the anterior coelom (=axocoel). In the roots, the central canal contains a root nerve, but no extensions of the haemal channel or of the coelomic tubes; therefore, roots of bourgueticrinid sea lilies are not homologous to cirri of isocrinid sea lilies or feather stars. The chambered organ and axial organ of D. conifer closely resemble the same organs in feather stars that have previously been described by electron microscopy. The functional implications of our structural results are: (1) cells in the stereom space appear to be a major site of nutrient reserves, (2) the abundant cells with lipid-rich organelles could make the sea lily body more buoyant, and (3) the absence of muscles or other cells specialized for contractility indicates that the stalk of bourgueticrinid sea lilies cannot bend actively.