ИСТИНА

Until recently, Pycnogonida were thought to lack specialized excretory organs. However, in 2007 coxal glands in cheliphores of Nymphopsis spinosissimum (Ammotheidae) were described. Here, we describe other examples of excretory organs in Pycnogonida. In Nymphon species (Nymphonidae) coxal glands are present in the fourth segment of the oviger; additionally, transitory coxal glands were found in basal podomeres of developing palp. Protonymphon larvae possess coxal glands in basal segments of larval legs at least in some Nymphon species. During the further development, the larval legs, including coxal glands reduce, and get replaced by definitive palps and ovigers; definitive coxal glands develop de novo. In Phoxichilidium femoratum (Phoxichilidiidae) protonymphon has coxal glands in basal segments of larval legs. Larval legs reduce in subsequent endoparasitic instars, nevertheless, coxal glands preserve in palp rudiments throughout the whole lifecycle. The structure of coxal glands is similar in all the pycnogonids examined: the sacculus, the reabsorption channel, and ectodermal duct, lined with cuticle and opening by an excretory pore. The sacculus wall consists of podocytes, possesses all the ultrastructural characters of a filtration site, and has signs of epithelization. Developmentally, podocytes of the definitive sacculus differentiate from unpolarized mesoderm of horizontal septa; the cavity of the sacculus forms by schizocoely. Thus, the cavity of the sacculus can be considered a secondary one. Altogether, to date, coxal glands were found in three pycnogonid families. They are localized in limbs, as well as many other pycnogonid organs. In different pycnogonids, the coxal glands are positioned in various segments of cephalosoma. The anterior localization can be explained by higher hemolymph pressure facilitating ultrafiltration in the area. The localization in different segments is related to the diverse fate of cephalic limbs in pycnogonid subtaxa and supports the hypothesis about the presence of serial segmentally homologous podocytary organs in the Chelicerata/Arthropoda urbauplan.