Functional morphology of the venom apparatus of Prorhynchus stagnalis (Platyhelminthes, Lecithoepitheliata)

The microturbellarian Prorhynchus stagnalis quells prey with a venom apparatus that also serves as its copulatory organ when the animal is mature. While it has long been known to comprise a tubular, sharp-pointed stylet surrounded concentrically by two mantles of longitudinally aligned rods, our examination of it by electron microscopy and confocal microscopy reveal unexpected details of its structure and mode of operation. The stylet actually has two distinct pieces, of two kinds of extracellular matrix (ECM): a sheet rolled into a tube with a sharp-pointed tip, and a sheet folded into a torus that fits into the proximal end of the tube to create a broadened base on which muscles and the male canal insert. The supporting rods are yet other phases of ECM in the mesenchyme. The six inner ones merge distally to ensheath the stylet, and that sheath bears five channels conducting the secretions of gland cells flanking the stylet to minute pores around the stylet tip. Confocal microscopy of the stylet apparatus in different phases of protrusion show that the outer rods are retracted from a protective position around the tip as the apparatus is protruded by protractors. The stylet is likely then protruded farther by contraction of the penis bulb, so that the stylet pore is exposed to allow the flow of secretions from the prostatic vesicle. The venom could be those secretions, as has been assumed, or secretions of the newly discovered gland cells of the inner mantle.