Trematodes nervous system.

Three main themes are discussed: 1. Development of the nervous system in the ontogeny of digenetic trematodes; 2. Central and integumentary nervous elements in Cercaria echinata (Echinostomatidae), their evolution in metacercaria and adult; 3. Chaetotaxy and systematics. The structure of the nervous system in digenetic trematodes is based on the occurrence of various neurotransmitters in the nerve tissue. Two major trends in the evolution of the nervous system from cercaria to adult are discerned, one leading to definitive, small number of commissures, established early in the development of the cercaria, the other one leading to formation of great number of commissures completed in the adult. Structure of the nervous system in sporocystic and/or redial generations in various groups of trematodes is compared. Its primitiveness in relation to that of other generations is noted. The central nervous system of Cercaria echinata is pointed out by fixation in glioxylic acid and examination in green light fluorescence. The integumentary sensory elements observed in light microscope, SEM and transmission electron microscope reveal a large variety in structures; propositions are made about their functions. In the corresponding metacercaria, the anterior part of the central nervous system remains unchanged; meanwhile the posterior part increases; several integumentary sensory elements disappear and poorly developed structures, improve; in the adult, transformations are completed. In about 200 various species of cercariae, the sensory receptors are arranged in a conspicuous relation with the nervous system. The study of their number and position; the chaetotaxy is used in systematics and allows very accurate conclusions in specific or infraspecific identifications and in all high taxonomical levels: family, sub-order, super-order. In the same way, miracidial and redial chaetotaxy are used n taxonomy.