ULTRASTRUCTURE OF THE CEPHALIC SENSORY ORGANS OF ADULT PYCNOPHYES DENTATUS AND OF THE FIRST JUVENILE STAGE OF P-KIELENSIS (KINORHYNCHA, HOMALORHAGIDA)

The ultrastructure of the paired cephalic sensory organs of adult Pycn ophyes dentatus and of the first juvenile stage of P. kielensis (Kinor hyncha, Homalorhagida) was investigated by TEM. In both species, each sensory organ is composed of one receptor cell and one en veloping cel l which border a common intercellular lu men. A single receptor cilium extends from the receptor cell into this lumen. The cilium expands be hind the basal body and branches into numerous processes. A pair of ce phalic sensory organs with these characteristics be longs to the groun d pattern of, at least, the Pycnophyidae. The sensory organs of these Kinorhyncha correspond closely with the anterior cephalic organs of th e Gastrotricha, but differ from the known cephalic receptors of other Nemathelminthes. Currently, it cannot be evaluated conclusively whethe r the last common ancestor of the Nemathelminthes possessed cephalic s ensory organs and, if it did, what these organs looked like.