rhe sicula ot Kozlowskitubus erraticus has been examined with TEM. The upper portion ot prosicula is three-layered, the middle layer being a mesh. Its bottom consists ot an attenuated, almost structureless membrane. The metasicula is made of strongly variable tusell!, built ot tusellar and sheet tabric only (Dictyonema type ot fuselli). The evidence available indicates that mesh tabric is the main component in all discophorous and in part ot nematophorous prosiculae (structural type "A"). Only some nematophorous prosiculae are lacking the mesh component and therefore constitute structural type "B". The striking similarity ot the sicula described with the embryonal vesicle ot Rhabdopleura may not be evaluated until more becomes known on latter's ultrastructure.