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The epidermis cf Lampetra spp. contains several kinds of differentiated cell; one innervated variety is characterized by bearing a group of large apical microvilli which project from the surface of the skin. In Lampetra planeri such oligovillous cells are numerous under the oral hood of the ammocoete larva, on the papillae fringing the dorsal fin and bordering the gill vents of the adult, and at the tip of the male genital papilla. Elsewhere on the head, body and fins they are present but more scattered, which appears to be the condition also in adult anadromous Lampetra fluviatilis. There are differences in the number and dimensions of the microvilli found on oligovillous cells, but each is supported by a stout core of actin filaments extending a variable distance down into the cytoplasm. Under the apex of the cell there are microtubules and numerous vesicles which are thought to be concerned in the renewal of the membrane on the microvilli. Beside and proximal to the nucleus is a system of channels of rough endoplasmic reticulum, and a stack of membranous cisternae which appears to have been derived from the endoplasmic reticulum. A nerve fibre is associated with the base of the cell which is indented by a spur-like process from the neurite. Typical “synaptic vesicles” are not found in the cell but irregular vesicular or canalicular profiles are associated with the cell membrane adjoining the neurite spur. The space between the cell and neurite membranes contains extracellular material with a characteristic appearance of prickle-like densities on the cell side meeting densities on the neurite membrane. Variations in the cytology of oligovillous cells can be explained in terms of a cycle of development and de-differentiation. Certain cells with vesicles throughout the cytoplasm and with a narrow apex without microvilli are interpreted as degenerate examples. The oligovillous cells are thought to be chemosensory receptors.