Fine structure and evolutionary significance of sagittocysts of Convolutriloba longifissura (Acoela, Platyhelminthes)

Sagittocysts are extrusomes found only in acoel turbellarians. They are nee dlelike secretory products, on the order of 18-50 mu m long and 1-5 mu m wi de, and consist of a fibrous cortex, a central filament, and an intermediat e lucent layer. We discovered sagittocysts in Convolutriloba longifissura, for which they had not been known before, by using confocal microscopy and a phalloidin-conjugated fluorescent stain that strongly labeled a mantle of muscle around the distal neck of the sagittocyst-secreting cell, the sagit tocyte. The muscle mantle apparently plays a role in ejecting the sagittocy st. Positions of the sagittocysts revealed by confocal microscopy suggests their role in defense as well as in prey capture. By electron microscopy, t he differentiation of sagittocysts was evident in the proximal part of the sagittocyte. The muscle mantle on the neck of the sagittocyte is conical in shape; its ribbon-like myocyte enwraps the neck in a tight spiral. Extrusi on of sagittocysts could be induced by stimulation with electrical pulses, light pulses, or weak hydrochloric acid; only the whole sagittocyst was eje cted, not its central filament. The presence of sagittocysts in species of Convolutriloba is sufficient to reassign the genus to the family Sagittifer idae. We establish the new subfamily Convolutrilobinae, with 3 species in t he genus Convolutriloba, and new subfamilies for the remaining sagittiferid s as well. As extrusomes, sagittocysts are comparable to nematocysts, collo blasts, rhabdites, and other extrusomes common especially to lower eumetazo ans, and the origin of all such extrusomes may correlate with the origin of the eumetazoan gut.