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The ultrastructure of the paired nerves, previously called frontal organ or X-organ, in copepod crustaceans was investigated. These nerves, running from the anterior margin of the brain to the frontal edge of the animals, are found to contain the dendrites of three types of morpho-logically different sensory neurons. The first unit consists of two dendrites (distinguished by their myelinization) leading to two small hairs on the front. Their detailed structure was not investigated. The second unit consists of a few large dendrites ending in branching cilia. The latter are surrounded by a specialized glial cell. The ciliary branches are regularly sized and arranged. The third unit consists of c. 17 dendrites ending with cilia at the cuticle. The cilia are split into irregular branches which are buried in modified epidermal cells which, in the case of Calanus, are connected with cuticular pores. By analogy with other presumed chemosensory organs in the Arthropoda, the second and the third unit are considered, on a morphological basis, to be chemoreceptors. The second unit receives internal stimuli. Because it resembles other X-organs in the Crustacea, all X-organs could have the same function. The third unit is thought of as receiving external stimuli. [ABSTRACT FROM AUTHOR]