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

Abstract

1. Amphioplus abditus has a vestigial two-piece larval skeleton that has portions with different crystallographic orientations. The larval skeleton is resorbed and, unlike that of echinoids, it does not act as a center of formation of the plates of the adult. The major skeletal elements of the adult develop from single (usually triradiate) spicules, and there is a uniform crystallographic orientation within each plate. 2. The radial shields, adoral shields, genital plates and genital scales are ophiuroid specializations without homologues in the asteroids. Ophiuroids can regenerate radial shields but not the apical primary plates (the latter are probably atavistic structures). 3. The madreporite and oral plates, generally thought to migrate from the dorsal surface of the disc, originate in situ on the ventral surface of A. abditus. A dorsolateral plate, probably confused with the madreporite in past studies, is a precociously formed interradial-1. The formation of a "precocious interradial plate" could be a vestige of the primitive ophiuroid madreporite. In fact, the madreporites of asteroids, ancient ophiuroids, and recent ophuroids may not be homologous. 4. The origin of each of the oral papillae is described. Buccal scales, previously (and incorrectly) thought to develop into peristomial plates, form the second oral papillae in A. abditus. Consequently, the second oral papillae of amphiurids should not be considered "oral tentacle scales". The true tentacle scales are cryptic structures within the buccal cavity. 5. The oral papillae of the different amphiurid genera are probably homologous. Judging from differences in the oral frame, there are probably two major amphiurid groups composed 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. 6. 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 first modified arm-segment and buccal scales may be the first pair of adambulacrals. The distal oral plates (ambulacral), adoral shields (adambulacral), and the first ventral arm-plate (within the buccal slit) compose the second transformed arm-segment of the oral frame. This pattern of homology, together with the dissimilarities between ophiuroid and asteroid discs constitute important differences between the ophiuroids and asteroids.