Abstract
The lack of mesocoelomic pores and the existence of a stone canal connecting proto-and mesocoel are characteristic peculiarities of echinoderms in contrast with the situation in other trimeric archicoelomates. The ultrastructure of the stone canal has been studied in order to understand its function in the strategically important position between two coelomic spaces with different functions. The epithelium of the Eucidaris (Echinoidea) stone canal is composed of three cell types: (1) ciliated cells, (2) cells with long basal processes containing myofilaments, and (3) granulated cells, which may represent secretory neurons. Nerve fibres of two types are common in the epithelium. We consider the stone canal to be a structure controlling fluid transport; its wall may exert peristaltic movements or tonic contractions and dilations which are under control of the nervous system. The ciliated cells additionally may have phagocytotic capacities. Similarities with the fine structure of the wall of the mesocoelomic pores in the pterobranch Cephalodiscus are discussed.
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