I took a sample of these filaments from another small mushroom polyp and put it under the microscope. Under a 4x objective (width of photo represents 5.5mm) this is how it looks like.
The dark dots are zooxanthellae (algae), the elliptical shapes with curled up strings inside are nematocysts.
A closer view with a 20x objective (width of photo represents about 1.1mm) under darkfield illumination results in this image.
The zooxanthellae are now shown in their natural brown colour. Large and small nematocysts are visible, some still embedded within the mesenterial filament.
Going closer still, this time with a 40x objective under DIC illumination (the width of the photo is approximately 0.54mm), the large elliptical nematocysts are still embedded in the mesenterial filament, with one about to burst out. The wound up filament inside is the threaded barb that will penetrate the target when ejected (these are not ejected).
This thread is surprisingly long. Here is a stacked image of a nematocyst that have been triggered. (The small nematocyst is still untriggered). A lone zooxanthellae cell is present just below the nematocyst.
The length of the triggered threads are partially shown here - they extend beyond the image boundaries.
More untriggered nematocysts.
