Scanning and transmission electron microscope studies on the sensory sucker papillae of the fish parasite Entobdella soleae (monogenea)

Summary The ventral surface of the posterior sucker of Entobdella soleae, a monogenean skin parasite of the sole Solea solea, is covered with more than 800 papillae ranging in size from 2.5–19 m in diameter. The papillae are penetrated by nerves which double over themselves to form a stack-like array of lamellae running parallel to the surface of the haptor. No cilia or associated structures are present within these presumed sense organs and the papillae have no opening to the exterior. The much larger sucker papillae of a related species, E. hippoglossi, have been shown to have a similar ultrastructure. The possibility that the papillae may be contact receptors or strain receptors, providing proprioceptive information assisting the coordination of the attachment organ, is discussed and the papillae are compared with vertebrate touch receptors.I should like to thank Colin Ogden of the British Museum of Natural History for the generous assistance with the scanning electron microscopy and the Director and Staff of the Plymouth Laboratory and Ken Mackenzie and Rod Wootten of the Marine Laboratory, Aberdeen for help in obtaining material.