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Fine Structure and Evolutionary Significance of Sagittocysts of Convolutriloba longifissura (Acoela, Platyhelminthes)

Robert Gschwentner, Peter Ladurner, Willi Salvenmoser, Reinhard Rieger and Seth Tyler
Invertebrate Biology
Vol. 118, No. 4 (Autumn, 1999), pp. 332-345
Published by: Wiley on behalf of American Microscopical Society
DOI: 10.2307/3227005
Stable URL: http://www.jstor.org/stable/3227005
Page Count: 14
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Fine Structure and Evolutionary Significance of Sagittocysts of Convolutriloba longifissura (Acoela, Platyhelminthes)
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Abstract

Sagittocysts are extrusomes found only in acoel turbellarians. They are needlelike secretory products, on the order of 18-50 µm long and 1-5 µm wide, and consist of a fibrous cortex, a central filament, and an intermediate 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 sagittocyte. The muscle mantle apparently plays a role in ejecting the sagittocyst. Positions of the sagittocysts revealed by confocal microscopy suggests their role in defense as well as in prey capture. By electron microscopy, the 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. Extrusion of sagittocysts could be induced by stimulation with electrical pulses, light pulses, or weak hydrochloric acid; only the whole sagittocyst was ejected, not its central filament. The presence of sagittocysts in species of Convolutriloba is sufficient to reassign the genus to the family Sagittiferidae. We establish the new subfamily Convolutrilobinae, with 3 species in the genus Convolutriloba, and new subfamilies for the remaining sagittiferids as well. As extrusomes, sagittocysts are comparable to nematocysts, colloblasts, rhabdites, and other extrusomes common especially to lower eumetazoans, and the origin of all such extrusomes may correlate with the origin of the eumetazoan gut.

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