DEVELOPMENT OF AMPHIOPLUS ABDITUS (VERRILL) (ECHINODERMATA: OPHIUROIDEA). II. DESCRIPTION AND DISCUSSION OF OPHIUROID SKELETAL ONTOGENY AND HOMOLOGIES

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Abstract
A. abditus has a larva with a vestigial 2-piece larval skeleton that has portions with different crystallographic orientations. Resorption of the larval skeleton and the chronology of skeletal element development in adults is described. Probably, the apical plates of ophiuroids and echinoids are not homologous, nor are the madreporites of asteroids, ancient ophiuroids and recent ophiuroids. The origin of each oral papilla is described. Buccal scales, previously (and incorrectly) thought to develop into peristomial plates, form the 2nd oral papillae. Judging from differences in the oral frame, there are probably 2 major amphiurid groups comprised of taxa which retain the buccal scales as oral papillae (Amphioplus and possibly Amphiura), and those like Axiognathus (and possibly Amphipholis and Amphiodia) which resorb the buccal scales. A new system of homologues is suggested for the plates of the ophiuroid oral skeleton. The proximal oral plate is considered the ambulacral portion of the 1st modified arm segment and buccal scales may be the 1st pair of adambulacrals. The distal oral plates (ambulacral), adoral shields (adambulacral) and the 1st ventral arm plate (within the buccal slit) compose the 2nd transformed arm segment of the oral frame. This pattern of homologies, together with the dissimilarities between ophiuroid and asteroid discs constitute important differences between the ophiuroids and asteroids.

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