STYLET FORMATION IN NEMERTEANS

¹ Department of Zoology, University of Washington, Seattle, WA 98195
² Friday Harbor Laboratories, Friday Harbor WA 98250

Stylet formation was examined in nine species of nemerteans by light microscopy. The first stylets produced by larvae are assembled intracellularly over a period of several days within styletocytes of reserve stylet sacs. A reserve stylet is moved to the lumen of the proboscis, apparently by muscular contractions, and placed on the basis to become the central stylet. The stylets of adult nemerteans are also formed in styletocytes of reserve stylet sacs, and, depending on the species, reach full length in two to eight weeks. At the onset of styletogenesis, a membrane-bound vacuole develops in the styletocyte, and an organic matrix is formed at one edge of the vacuole. The calcified cortex of the stylet shaft is subsequently deposited around the organic matrix, and a knob-shaped proximal piece is formed on the shaft. Most adult nemerteans contain at least one developing stylet, and the rate of stylet formation is about the same in starved worms as it is in worms that have recently captured prey. Replacement of the central stylet occurs following prey attack, and occasionally when the worm is not feeding.