Fine Structure of the Integumentary Cuticles and Alimentary Tissues of Pycnophyid Kinorhynchs Pycnophyes oshoroensis and Kinorhynchus yushini (Kinorhyncha, Homalorhagida).

Integumentary and alimentary tissues were ultrastructurally examined in two pycnophyid kinorhynchs, Pycnophyes oshoroensis and Kinorhynchus yushini, to elucidate some aspects of their ecology. The body is entirely enveloped by an epicuticle layer with no gaps between cuticle plates and joints. The cuticular layer has a structure dense enough to prevent invasion by foreign organisms. The cuticular surface is overlaid by a mucus layer that may form a hydrophilic surface. The alimentary contents were heterogeneous, probably including some cellular components, such as chloroplast-like structures. Kinorhynchs likely break down food particles in the pharyngeal bulb by pressing it between the cuticulated epithelia. The pharyngeal crown was located in front of the pharyngeal bulb and had a thick wall with a striated sub-structure. Contraction of the pharyngeal bulb probably increases the internal pressure of the pharyngeal crown; this may be one reason for the thick wall of the pharyngeal crown. Nutrients appear to be taken up by midgut epithelial cells through both absorption via microvilli and endocytosis. Additionally, sperm tails in the testis of P. oshoroensis have unusual axonemes; i.e., an 18+9+2 pattern.