ИСТИНА

Ultrastructure of pycnogonida larvae has not been heretofore described. Ultrastructure of the protonymphon larvae of Nymphon brevirostre and Phoxichilidium femoratum is the subject of the present study focused on the sense organs and neuroectoderm. The series of developing neuromeres represents sequential phases of ganglion formation. The so-called “ventral organs” are the sites of the cell internalization in the neuroectoderm corresponding to each hemineuromere. Ultrastructure of the ectoderm and neuroectoderm cells, mitotic and post-mitotic precursors of the ganglion cells, and immigrating cells of the forming ganglia is described. Many of the cells of the apical cell layer of “ventral organs” have cilia. Cilia are also found in the ectoderm out of the ventral organs. In some cases the short and presumably immovable cilia have no evident relation to any sense organs. The longer cilia belong to sensilla common for adult pycnogonids and larvae. Sensory, apparently mechanoreceptory, function may be ascribed to the terminal thread-like segments of the larval limbs 2-3 of Phoxichilidium protonymphon on the ground of presence of ciliary cells in their internal structure. The spine on the basal segment of larval limb 2-3 has cilia inside. This spine is a sensory organ and not a serial homologue of the spinning spine as was considered before. Instead, a presumable serial homologue of the spinning apparatus is decsribed in Phoxichilidium larva. This organ is in the basal segment of the larval limbs 2-3 larvae and its opening is next to the base of the spine. Larval simple eyes basically resemble the eyes of adult specimens but have no evident cuticle lenses and consist of only a few cells. An eye is composed of retinula cells with a proximal rhabdomeric zone and a layer off cells containing pigment granules in the basal zone and reflecting crystals in the apical zone.