Fine structure of the stalk of an isocrinid sea lily (Metacrinus rotundus) (Echinodermata, Crinoidea)

Summary

Sea lilies of the order Isocrinida (Metacrinus rotundus) were dredged from Sagami Bay, Japan, and their stalks were studied by transmission electron microscopy. New contributions to stalk histology are: an exact description of the different cell types in the stereom spaces; a demonstration of the haemal channel; and the discovery of (1) coelomic nerves, (2) collateral stalk nerves and (3) nerve tracts running with the aboral extension of the axial organ. Within the stalk, there is no structure derived from the axial sinus (=axocoel), and the widely accepted homology between the crinoid stalk and the larval asteroid stalk is thus open to serious question. The stalk has collagenous ligaments of two main types (intercolumnal ligaments and peripheral through-going ligaments); however, there are no central through-going ligaments comparable to those in stalks of larval feather stars and of bourgueticrinid sea lilies. The absence of muscles or other cells specialized for contractility indicates that the stalk of isocrinid sea lilies cannot bend actively. The cirri, which project from the stalk of M. rotundus, contain no muscle cells; however, the epithelial cells lining the oral and aboral coeloms of each cirrus contain a presumed contractile apparatus that is a bundle of 5 nm cytoplasmic filaments oriented parallel to each other and to the long axis of the cirrus.