ИСТИНА

The nervous system (NS) investigation has a long history, and comparison of its organization in different taxa helps greatly in understanding of Metazoa evolution. At the same time, the organization of the NS can vary between representatives of the same taxon according to the ecology of species. Larval NS participates significantly in searching for the place of settlement and triggering of metamorphosis. At present, it is obvious that there are several types of NS organization in cnidarian larvae. The NS of hydrozoan larvae is relatively simply organised and represented by the sub-epidermal plexus formed by the processes of the sensory, ganglionic and neurosecretory cells. Comparison of the larvae of different hydrozoan species reveals certain variations in NS organization especially of its part revealed by anty-FMRF-amide antibodies. In some larvae the double-labeling for acetylated and tyronisated tubulin reveals the sub-epidermal plexus composed of relatively thick longitudinal and transversal (circular) nerve bundles. Most of these bundles include also FMRF-amide positive cell processes. The larvae with such NS organization represent species that lost free medusa stage and develop within sessile gonophores. The planula larvae of the species with free medusa stage in the life cycle and developing in the water column, display somewhat different organization of the NS. Their nervous plexus has higher order of organisation due to the more regular arrangement of the transversal (circular) nerve bundles. FMRF-amide immunoreactivity is detectable only within the longitudinal nerve bundles. Such organization of the FMRF-amide positive NS resembles those of the scyphozoan larvae, which also leaves maternal organism spending long time in the water column. By the way, the anti-tubilin antibodies do not reveal any nerve structures in the scyphozoan larva. We suppose that such differences in the NS organization can concern with the differences in the larval mode of life. Larvae of the first group develop in the sessile gonophores of the benthic colonies and after release move predominantly along the substrate. Larvae of the second group that develops in the water column or in medusoids (meconidia), spend long time in the water column after releasing approaching substrate only shortly before attachment and metamorphosis.