POSTEMBRYONIC DEVELOPMENT OF PARACENTROPHYES PRAEDICTUS (HOMALORHAGIDA) - NEOTENY QUESTIONABLE AMONG THE KINORHYNCHA

The postembryonic ontogeny of the kinorhynch Paracentrophyes praedictu s is described by light microscopy and scanning electron microscopy. T he development of head scalids and trunk characteristics has been stud ied. The first of 6 juvenile stages possesses 11 segments already. One segment each is added both in stages 2 and 5. The number of head scal ids increases during development. Every scalid becomes visible as a sp inose anlage in one stage and differentiates in the next stage. All he ad scalids in stage 1 represent anlagen of scalids. Late juvenile stag es J-5 and J-6 express two different scalid patterns which do not seem to depend on the gender. Scalids and trunk segments develop in a subf rontal and a subcaudal growing zone, respectively. The presumed neoten ic status of P. praedictus is rejected because this species exhibits t he same number of juvenile stages as observed in all other Kinorhyncha studied. It is suggested that a weakly sclerotized, juvenile-like tru nk cuticle as found, e.g. in adult P. praedictus represents the plesio morphic characteristic within the Kinorhyncha; a thickened cuticle has probably evolved independently within both the cyclorhagid and the ho malorhagid Kinorhyncha.