ATYPICAL CHORDOID STRUCTURES IN THE ARISTOTLES LANTERN OF REGULAR ECHINOIDS

Regular sea-urchins, as a rule, prepare and process their food as pell ets in their buccal cavity before ingesting them. This is obtained thr ough the synergistic interaction of the five teeth with five fleshy pr otuberances (paradental tongues) which extend into the oral cavity and work like tongues. Each of these structures consists of two anatomica lly distinct parts: the outer component is a deep pouch of the termina l tract of the pharynx and shows a histological organization similar t o that usually shown by the pharyngeal wall itself. The deeper compone nt (paradental axis) is represented by a supporting rod and shows a pe culiar histological structure, which consists of glycogen containing v esiculate cells mixed to slim muscle fibres and held together by a thi n fibrillar stroma. The paradental axes are very problematic structure s in terms of comparative anatomy and phylogenetic aspects. Their micr oscopic and submicroscopic organization is unusual for an echinoderm a nd recalls that of different types of chordoid organs commonly found i n other invertebrates (Protostomata and Deuterostomata).