Complex sentence: The shells have discoidal nuclear whorls that are shed soon after the adult stage commences, which is in the form of a simple slightly curved tube, now sealed off above by a blunt rounded septum.
Split into simpler sentences: The shells have discoidal nuclear whorls that are shed soon after the adult stage commences. The adult stage is in the form of a simple, slightly curved, tube, which is sealed off at the narrow end by a blunt rounded septum.