Non-respiratory podia of clypeasteroids (Echinodermata, Echinoides)

Abstract

The podia found in the Order Clypeasteroida are composed of three basic systems: (1) a specialized external epithelium, (2) a connective tissue sheath enclosing a hydrostatic skeleton, (3) an intrinsic smooth muscle system. The components of each system are described using histology and scanning electron microscopy and their particular functions are elucidated. The external epithelium is specialized at the tip and supports short cilia on the disk and diaphragm, and long cilia on the periphery of the disk. The epidermis is covered by a mucous layer (approximately 1 μm), that is studded by microvilli-like projections. Two types of secretory cells are present in the tips of all suckered podia. Small secretory cells amongst the disk muscles produce adhesive substances. Large secretory cells of the podial disk periphery are described in the Clypeasteroida for the first time. Secretions from these cells are used during sucker attachment, and as binding material for food particles. Dissections of podia display the arrangement of fibres that constitute the collagenous connective tissue sheath and its relationship to the podial nerve. Longitudinal connective tissue is found in three bundles running the length of the podium. Circular connective tissue forms a continuous sheath separating lumen musculature and coelomic epithelium from the external epithelium. Disk and radial connective tissue fibres maintain the shape of the tip and help transmit forces from the various podial muscles. Spicules perform a similar function. Stem retractor and levator muscles are found just inside the connective tissue sheath. Disk and diaphragm muscles outside the sheath (along with the levators) operate the tip during particle manipulations. Descriptions of these activities, based on live observations and scanning electron microscope studies of mucous prints, are provided. Podia develop as heavily nucleated buds of epithelium growing at the exits of podial canals through the test. The tip and connective tissues differentiate before the nervous tissue, muscles, and secretory cells. Clypeasteroid suckered podia are shown to be most similar to those found in the Order Cassiduloida.