ULTRASTRUCTURE OF THE INTROVERT AND ASSOCIATED STRUCTURES OF THE LARVAE OF HALICRYPTUS-SPINULOSUS (PRIAPULIDA)

Based on new SEM and TEM observations, the arrangement and ultrastruct ure of the scalids and adhesive tubuli of the larvae of Halicryptus sp inulosus are described. The scalids are arranged in 25 longitudinal ro ws. Transversally they form seven circlets which alternate. The first circlet consists of 8 and the second of 9 scalids. All following circl ets consist of three subrings with 8 + 8 + 9 scalids each. New scalids develop in a sub frontal growing zone posterior to the second circlet . The ultrastructure of all different receptor cells and sense organs of the scalids is described. Whereas the receptor cell type 1 occurs i n nearly all scalids, the vertical-rootlet receptor is restricted to t he first circlet, the basal receptor to the second and the coiled-cili um receptor to the third and fourth circlet. Based on these new data, the scalid arrangement and the equipment with different receptor cells can be postulated for the ground pattern of the priapulid larvae. Als o the larval development, moulting and the structure of the cuticle ar e discussed in comparison with the Kinorhyncha and Loricifera.