ИСТИНА

The most significant features which characterize diverse variants of postembryoinc development of Pycnogonida are: 1) the first postembryonic stage; 2) order of development of the walking legs; 3) mode of feeding during larval period (lecitotrophic or active feeding as ecto- or endoparasites) which correlates with the egg size. Lecitotrophic development may start with a protonymphon stage but a strong tendency towards the embryonization of the early larval stages has been traced in Ammotheidae, Callipallenidae and Nymphonidae and is especially characteristic for Arctic and Antarctic pycnogonids. In callipallenids protonymphon larvae are absent and larvae hatch at more advanced stage with two or even three pairs of the walking legs anlagen (Brennies et al., 2011). Ammotheidae demonstrate a broad spectrum of modes of postembryonic development. Recent studies revealed that in many Antarctic ammotheids lecitotrophic larvae hatch as very large protonymphons, lack spinning apparatus and show sequential pattern of development of the walking legs (Cano, Lopez-Gonzalez, 2013). So, ammotheids do not show embryonization and their lecitotrophic development and larvae differ from that of callipallenids. For Nymphonidae protonymphon larva is common as the first stage of non-lecitotriphoc and lecitotrophic larval development. New original data include description or re-description of larvae of four Nymphon species. The development of Arctic N. helleri Bohm, 1879 starts with protonymphon of the size and external morphology typical for the ectoparasitic actively feeding larvae. Other three species have lecitotrophic larvae. Development of Arctic N. sluiteri Hoek, 1901 is characterized by the sequential development of the walking legs and the presence of typical larval apendages II and III which implies existence of protonymphon larva. Arctic N. hirtipes Bell, 1853 shows signs of embryonization of the protonymphon stage; the earliest of found instars has larval limbs II, III and anlagen of walking legs I. The most interesting is development of Antarctic N. australe Hodgson, 1902. Protonymphon stage absent and the larvae hatch having three pairs of the walking legs anlagen. This pattern of development is a unique example of embryonized larvae in a nymphonid species reported for the first time since 19th century. Adding this to the previous data it is possible to make a sequence of embryonization of early larval stages within Nymphonidae similar to that for Callipallenidae which conforms to current concept of pycnogonid phylogeny. In ammotheids lecitotrophic development might have evolved convergently. An approach to developing terminology for postembryonic stages, types of larvae and patterns of development is suggested.