Comparative studies on receptor structure in the brittlestarOphiura ophiura

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Abstract
The morphology of cells incorporating modified cilia in the epithelium of the spines, tube-feet and general surface of the ophiuroid echinodermOphiura ophiura are described using transmission and scanning electron microscopy and compared to those on the tube-feet of the echinoidEchinus esculentus. Some receptor cells have cilia showing a normal 9 + 2 microtubular arrangement and these project from the cuticular surface. There are also three basic cell types in which the cilia do not show a continuous normal 9 + 2 microtubular arrangement. In one class, a ring of microvilli form a tube which protrudes from the cuticular surface and which contains a cilium. This type is present on both spines and tube-feet and is identical in structure to the Stäbchen receptors described previously. The second type, termed the non-projecting receptor, is not surrounded by specialized microvilli, and the cilium which projects from the receptor cell ends flush with the cuticle which may be 3 or 4 μm above the epithelial surface; these are only present on the tube-feet. The third type of receptor occurs on the general surface of the arm, and from its position and some aspects of its structure may be a general photic receptor. It consists of a highly modified cilium characterized by a very short ciliary root. The cilium projects very little above the receptor cell surface and terminates among the extracellular fibrillar material that packs the gap of variable width between the epithelial cellular surface and the non-cellular cuticle. The distribution of these presumptive receptor cells is discussed in the light of recent sensory physiological findings and previous descriptions by other authors of receptor structures in the whole phylum.