ИСТИНА

Little is known about the excretory system of Pycnogonida which is of particular interest due to the phylogenetic position of pycnogonids as the sister group to Euchelicerata or to all other arthropods. TEM and SEM were used to search excretory organs in the protonymphon larva of Phoxichilidium femoratum (Rathke, 1799). The complex of three structures in the basal articles of the larval limbs 2 and 3 aroused most interest: 1) epidermal gland common for the integument of pycnogonids; 2) spine typical for protonymphon larvae; ultrustructure of the spine implies that it functions as a sensory seta; 3) a minute sac covered with the ECM, it has proximal wall composed of podocytic pedicels, narrow lumen and exit duct. The same organs were found in the larvae of different pycnogonids (Meisenheimer, 1902; Dogiel, 1913) and were initially interpreted as glands homologous with the spinning apparatus in the larval limbs 1 which secretes a silk thread. However, the ultrastructure of the sacs in the larval limbs 2-3 suggests ultrafiltration and reabsorption rather than secretion. Moreover, the organ in the larval limbs 2-3 has a separate opening not on the spine but next to its base. We consider the sacs in the limbs 2-3 of the protonymphon larvae as filtration nephridia. The only known example of nephridia in the limbs 1 of pycnogonids were found in adult Nymphopsis spinosissima (Fahrenbach, Arango, 2007). In all, in pycnogonids paired nephridia are found in three anterior limbs, which resemble other segmental excretory organs of panarthropods.