Non-respiratory podia of clypeasteroids (Echinodermata, Echinoides): II. Diversity

Abstract

There are five major types of non-respiratory podia in the Order Clypeasteroida: accessory, barrel-tipped, food groove, large food groove, and buccal. The anatomy of each type is intimately related to its function in the feeding mechanism of clypeasteroids. Accessory podia are found aborally and orally in some species, only aborally and ambitally in others. Accessory podia are largely sensory and manipulatory, but can be locomotory in the small fibulariids and juvenile sand dollars. Barrel-tipped podia have expanded disk muscles and connective tissue, and are usually found in two sizes, large and small. In species that have them they are usually restricted to the oral surface. These podia collect food and pass it towards the food grooves in the manner of a bucket brigade. Food groove podia are found only in species with food grooves. These podia are small, with reduced tip musculature and expanded secretory tissue for coating food with mucus. They transport food down the food grooves to the mouth. Large food groove podia are simply large versions of ordinary food groove podia. They help move the clumped food into the mouth area towards the buccals, and are found only in the Clypeasteridae and some scutellines. Buccal podia lack tip musculature, but possess tip support fibres and a single type of small secretory cell. They are sensory, and capable of manipulating particles into the mouth. Buccals are present in all families except the Clypeasteridae. Juvenile Echinarachnius less than 3 mm in diameter have only respiratory, accessory and buccal podia. Food groove and barrel-tipped podia start to differentiate from the accessories as the juvenile approaches a diameter of 4 to 5 mm. Clypeasteroid podial diversity increases the efficiency of the food collecting mechanism. The anatomy and distribution of podia on the oral surface of scutellines supports the fact that this surface is the prime food collecting area in all true sand dollars. The podia (not miliary spines) are the major source of mucus used during the feeding process and are the primary feeding appendages.