BIOLOGIYA MORYA, 2013, Vol. 39, No. 2, pp. 106-114

Morphology of the apical organ and adjacent epithelium of pilidium prorecurvatum, a pelagic larva of unknown heteronemertean (Nemertea)

© 2013 A. V. Chernyshev^{1, 2}, A. A. Astakhova³, S. Sh. Dautov¹, V. V. Yushin^{1, 2}

¹A.V. Zhirmunsky Institute of Marine Biology, Far East Branch, Russian Academy of Sciences, Vladivostok 690059;
²Far Eastern Federal University, Vladivostok 690600;
³Lomonosov Moscow State University, Moscow 119991

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The morphology of pilidium ex gr. recurvatum from Peter the Great Bay (Sea of Japan) was studied by laser scanning confocal and transmission electron microscopy. The studied pilidium larvae differ from pilidium recurvatum in lacking a posterior ciliary ring and in having a caudal tuft. On this basis, pilidium prorecurvatum is proposed as a new name for them. The apical organ of pilidium prorecurvatum is represented by a thickened epithelium, which consists of uniform columnar monociliary collar cells and is innervated by a pair of serotonergic intraepithelial neurons. The bodies of the serotonergic neurons are located outside the apical organ, but occasional axons were found at the organ base. The rest of the pilidial epithelium is represented by flattened polygonal multiciliated cells with sparse microvilli; the bodies of two neurons lie in the cupola epithelium immediately adjacent to the apical organ. Morphologically, the apical organ of the pilidium corresponds well to that of other lophotrochozoan larvae, but their homology remains unclear.

Key words: nemerteans, pilidium, apical organ, nervous system, ultrastructure, laser scanning confocal microscopy.