FROM COMPETENT LARVA TO EXOTROPHIC JUVENILE - A MORPHOFUNCTIONAL STUDY OF THE PERIMETAMORPHIC PERIOD OF PARACENTROTUS-LIVIDUS (ECHINODERMATA, ECHINOIDA)

The perimetamorphic period in Paracentrotus lividus lasts for 8-12 day s. It starts from the acquisition of larval competence, includes the c hange in form (metamorphosis) and the endotrophic postlarval life, and stops with the appearance of the exotrophic juvenile. All major postl arval appendages already occur in competent larvae being either groupe d into the echinoid rudiment (terminal plates, early spines and primar y podia) or scattered within the larval integument (genital plates and sessile pedicellariae). Competent larvae show particular behaviour wh ich brings them close to the substratum. The latter is tested by prima ry podia protruding through the vestibular aperture of the larva. Prim ary podia are sensory-secretory appendages that are deprived ampullae. They are able to adhere to the substratum in order to allow evaginati on of the echinoid rudiment (i.e. metamorphosis) and substatum adhesio n of the postlarva. Particular spines are borne by the postlarva; thes e are multifid non-mobile appendages forming a kind of protective armo ur. Like those of the larva, all characteristic structures of the post larva (primary podia, multified spines and sessile pedicellariae) are transitory and regress either at the end of postlarval life (primary p odia) or during early juvenile life (multifid spines and sessile pedic ellariae). Other appendages that develop during postlarval life (i.e. podia with ampulla, point-tipped spines and sphaeridiae) are similar t o those borne by the adults and become functional when the individual enters its juvenile life. Thus, the perimetamorphic period appears to be a fully fledged period in the life-cycle of P. lividus, and presuma bly in the life-cycle of any other sea-urchin species.