ABSTRACT

The clypeasteroid echinoid Laganum depressum possesses thousands of minute podia, the so-called accessory podia, which are distributed throughout the five ambulacra on both the oral and aboral surfaces. Scanning and transmission electron microscopy studies have revealed the complex ultrastructure of these podia. Their disc epidermis encloses 11 cell types: 6 types of secretory cells, 3 types of ciliated cells, support cells and covering cells. Accessory podia of clypeasteroid echinoids are involved in feeding and are generally recognized as handling and sensory. The podia of L. depressum have the right structure to fulfil these functions. The different types of secretory cells would allow the podium either to handle large substrate particles (adhesion would solely be due to adhesive secretions, not to a sucker-like operation of the disc) or to wrap together the smaller particles in a binding material to form an easily managed mass. The ciliated cells would be the site of sensory perception.