Structure and function of clypeasteroid miliary spines (Echinodermata, Echinoides)

Summary

Histological and ultrastructural techniques have been used to describe the functional morphology of clypeasteroid miliary spines, with special reference to their supposed mucus-secreting role. Mucus cells were not found in the miliary spines of any members of the Arachnoididae, Fibulariidae, Laganidae, Echinarachniidae, Dendrasteridae, Astriclypeidae, or Mellitidae examined in this study. Only members of the Clypeasteridae have mucus-secreting cells in these spines. Characteristics of the skeleton, ultrastructure of the nervous system, and histology of the musculature and epithelia of the base, shaft and tip are also discussed. Miliary spines have two bands of cilia running along the entire length of opposite sides of the shaft. The geometric packing of cilium-bearing cells in these bands is described for the first time, as is the remarkable form of the sacs found at the tips of dendrasterid, astriclypeid, and mellitid miliary spines. These sacs are definitely not “mucous sacs”, as previously described, but are balloons of single-celled epithelium internally tethered to the skeletal tip by copious quantities of collagenous connective tissue. Miliary spines prevent obstruction of aboral nutritive and ventilatory ciliary currents caused by substrate particles falling to the test surface during burrowing. They do this in two ways: (1) they help generate ciliary currents that sweep finer material off the test, and (2) they contribute to the formation of a spine canopy that mechanically blocks larger particles from falling between the spines. Members of the Clypeasteridae secrete an interspine mucous tent that traps potentially clogging material. The miliary spine sacs of sand dollars are deformable space-fillers that plug holes between primary spines in the aboral canopy, even as the spines rock on their tubercles to push sand backwards over the test. Allometry of spines from Echinarachnius parma suggests that aboral military spines and club-shaped spines exhibit co-ordinated growth that maintains the aboral canopy throughout post-metamorphic ontogeny, and that aboral spins have an overall lower growth rate than spines on the oral surface.