Ultrastructure of the cephalic sensory organs of adult Pycnophyes dentatus and of the first juvenile stage of P. kielensis (Kinorhyncha, Homalorhagida)

Abstract

 The ultrastructure of the paired cephalic sensory organs of adult Pycnophyes dentatus and of the first juvenile stage of P. kielensis (Kinorhyncha, Homalorhagida) was investigated by TEM. In both species, each sensory organ is composed of one receptor cell and one enveloping cell which border a common intercellular lumen. A single receptor cilium extends from the receptor cell into this lumen. The cilium expands behind the basal body and branches into numerous processes. A pair of cephalic sensory organs with these characteristics belongs to the ground pattern of, at least, the Pycnophyidae. The sensory organs of these Kinorhyncha correspond closely with the anterior cephalic organs of the Gastrotricha, but differ from the known cephalic receptors of other Nemathelminthes. Currently, it cannot be evaluated conclusively whether the last common ancestor of the Nemathelminthes possessed cephalic sensory organs and, if it did, what these organs looked like.