ULTRASTRUCTURE OF THE COELOMS OF AURICULARIA LARVAE (HOLOTHUROIDEA, ECHINODERMATA) - EVIDENCE FOR THE PRESENCE OF AN AXOCOEL

A hallmark feature of echinoderm larvae is the development of the left anterior coelom. This coelom, called the axohydrocoel, consists of th e morphologically distinct, but undivided, left axocoel and hydrocoel. The axocoelic portion forms a duct that opens to the exterior via a p ore on the dorsal surface of the animal. Holothuroid larvae are though t to lack an axocoel, but develop an.anterior coelom, duct, and pore t hat are regarded as parts of the hydrocoel. New ultrastructural data, however, show that holothuroid auricularia larvae possess an axocoel a nd hydrocoel united together into an axohydrocoel. During development the anterior coelom consists of an interconnected left somatocoel, hyd rocoel, and axocoel. The left somatocoel separates from the axohydroco el and subdivides into left and right somatocoels. The somatocoels and hydrocoel region of the axohydrocoel are lined by a monociliated meso thelium having characteristics of transporting epithelia. The axocoel epithelium, like that of asteroid larvae, is composed of mesothelial p odocytes. A duct connects the axocoel directly to the open dorsal pore and is lined with a columnar transporting epithelium. The occurrence of a specialized podocyte-lined cavity between the surface pore and th e hydrocoel in echinoderm larvae is indicative of an axocoel. That sim ilar structures occur in auricularia larvae supports the identificatio n of an axocoel in holothuroids.