Metamorphosis of the marine gastropod Phestilla Sibogae Bergh (nudibranchia: aeolidacea). I. Light and electron microscopic analysis of larval and metamorphic stages

The structure and fate of transitory larval organs (velum, shell, operculum, retractor muscles, part of the epidermis) of Phestilla sibogae Bergh were studied before, during, and after metamorphosis with both light and electron microscopy to elucidate the morphology of these organs and the mechanisms by which they are lost.Loss of the velar lobes is the first morphological sign of metamorphosis, and involves selective dissociation and subsequent ingestion of the ciliated velar cells; the remaining aggregate of supportive cells is apparently incorporated into cephalic epidermis. Attachment of the larval body to shell and operculum is primarily at sites of retractor muscle insertions; once the velum is gone, the attachment between shell and larval body is lost and the shell is cast off as the visceral organs exit through the shell aperture. Merger of visceral and cephalopedal elements results in flattening of the postlarval body and reorientation of internal organs. Simultaneously, a rapid spreading of epipodial epidermis over the lateral, dorsal, and posterior sides of the body produces the definitive integument. The squamous cells which comprise the larval perivisceral epidermis are pushed ahead of the definitive epidermis and are seen shortly after the shell is cast as a constricted aggregate of cells on the posterior end of the body. Autolysis of the left and right retractor muscles begins during metamorphosis and no trace of them is left after 24 to 48 h. The metapodial mucous glands which hypertrophy before metamorphosis are also lost within 48 h following exit of the post larva from the shell. Metamorphosis produces a detorsion caused in part by muscular action and in part by continuing growth and development.