ELECTRON-MICROSCOPIC LOCALIZATION OF CHITIN IN THE CUTICLE OF HALICRYPTUS-SPINULOSUS AND PRIAPULUS-CAUDATUS (PRIAPULIDA) USING GOLD-LABELEDWHEAT-GERM-AGGLUTININ - PHYLOGENETIC IMPLICATIONS FOR THE EVOLUTION OF THE CUTICLE WITHIN THE NEMATHELMINTHES

The ultrastructure of the cuticle of adult and larval Priapulus caudat us and Halicryptus spinulosus is investigated and new features of cuti cle formation during moulting are described. For the localization of c hitin by TEM wheat germ agglutinin coupled to colloidal gold was used as a marker. Proteinaceous layers of the cuticle are revealed by diges tion with pronase. The cuticle of larval and adult specimens of both s pecies consists of three main layers: the outer, very thin, electron-d ense epicuticle, the electron-dense exocuticle and the fibrillar, elec tron-lucent endocuticle. Depending on the body region, the exocuticle comprises two or three sublayers. The endocuticle can be subdivided in to two sublayers as well. In strengthened parts such as the teeth, the endocuticle becomes sclerotized and appears electron-dense. Only all endocuticular layers show an intense labelling with wheat germ aggluti nin-gold conjugates in all investigated specimens. Additional weak lab elling is observed in the exocuticle III layer of the larval lorica of P. caudatus. All Other cuticular layers remain unlabelled. Chitinase dissolves the unsclerotized endocuticular layers almost completely, bu t also exocuticle II and partly the loricate exocuticle III. The epicu ticle, the homogeneous exocuticle I and the sclerotized endocuticle ar e not affected by chitinase. The labelling is completely prevented in all layers after incubation with chitinase. Pronase dissolves all exoc uticular layers, but not evenly. The presumably sclerotized regions of exocuticle I are not affected as well as the complete epicuticle and the endocuticle. All cuticular features of the Priapulida are compared with the cuticle of each high-ranked taxon within the Nemathelminthes with special regard to the occurrence of chitin. Based on this out-gr oup comparison it can be concluded that: (I) a two-layered cuticle wit h a trilaminate epicuticle and a proteinaceous basal layer represents an autapomorphic feature of the Nemathelminthes, (2) the stem species of the Cycloneuralia have already evolved an additional basal chitinou s layer, (3) such a three-layered cuticle is maintained as a plesiomop hy in the ground pattern of the Scalidophora and (4) in the Nematoida, the chitinous basal layer is replaced by a collagenous one at least i n the adults; the synthesis of chitin is restricted to early developme ntal phases or the pharyngeal cuticle.