Larval and adult eyes in Capitella spec. I (Annelida, Polychaeta).

Larval and adult eyes of the capitellid Capitella spec. I have been investigated by electron microscopy. Developing larvae possess one pair of eyespots. Each eyespot is composed of one sensory cell, one pigment cell, and one supporting cell. The sensory cell bears a photosensory apparatus, an array of parallel microvilli containing a striking form of submicrovillar cisternae (SMC). A single basal body was observed near the basal end of the SMC only in 3-4-day-old larvae. This basal body was not observed in older larvae. A special, thus far unreported feature of the pigment cell is a ring of microvilli-like structures that project from the edge of the pigment cup and form a kind of diaphragm around the head of the sensory cell. Each of these microvilli-like structures is covered with closely packed rows of tiny plates, giving them a fimbriated appearance. After metamorphosis, the larval eyespots are reduced. Adult specimens possess one pair of eyes. In juvenile worms the eye is built of 2-3 pigment cells and one sensory cell. The volume of adult eyes increases by the addition of 2-3 sensory cells. The pigment cells are deposited one upon the other, wrapping the sensory complex in the center. Each sensory cell forms a deep depression at its apical end where up to three cilia and numerous microvilli arise. The sensory processes project into a common ocular cavity. Long microvilli of the uppermost pigment cells project into an extracellular channel that reaches from the ocular cavity up to the cuticle. A single fourth sensory cell of identical structure may occur next to the eye directly beneath the cuticle and close to the opening of the ocular channel. Signs of progressive disintegration of the pigment cells can be observed in adult specimens. Sensory cells are not affected by this reduction process. Eyes resembling those of Capitella spec. I have never been reported before. Their functional qualities are discussed. © 1993 Wiley-Liss, Inc.