The larva of the archiannelid Polygordius was studied using light microscopy (LM), scanning ( SEM) and transmission electron microscopy (TEM). The embryo of Polygordius develops into a large trochophore larva adapted to planktonic swimming and feeding. The young larva is characterized by the presence of an approximately hemispherical episphere including a basal prototroch and a well-developed apical organ which lacks a central ciliary tuft. The conical hyposphere is smaller and contains the metatrochophore segments. The apical organ of the episphere consists of a large, bulbous mass of cells that project deeply into the spacious blastocoel. Two tentacular knobs and two ciliary aggregations are located on the surface of the apical organ. The entire surface of the episphere is provided with scattered cilia. Two pigmented eyespots or ocelli are embedded in the uppermost region of the apical organ. A second presumed photoreceptor organ, the phaosome, is situated centrally between the two ocelli. The apical organ is covered by a thick cuticle penetrated by numerous microvilli. Ectodermal cells peripheral to the apical organ contain numerous vacuoles of flocculent, lightly staining material. Mucus cells are also present in this region of the episphere. An apical intraepidermal nerve plexus is located in the basal most region of the apical organ. Axons occur in the apical and basal regions of the apical organ. Morphological evidence supports the suggestion that the apical organ functions in chemoreception and/or mechanoreception related to substrate selection, metamorphosis and other planktonic behaviors.
Sensenbaugh, Terry and Franzén, Ake
"Fine Structural Observations of the Apical Organ in the Larva of Polygordius (Annelida: Polychaeta),"
Scanning Microscopy: Vol. 1
, Article 17.
Available at: https://digitalcommons.usu.edu/microscopy/vol1/iss1/17