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A small sample of Sagittozoa, specialized predator and parasitic worms. On the left: goliath arrowtooth (Phobodus delabens), a predator that lets itself fall on the prey from a great height. On the right, at the bottom: swimming dartleech (Machairobdella oxyops), a small aquatic parasite. On the right, at the top: variety of rostra.

«Tentatively placed in the phylum Nematozoa along with other simple, limbless Horozoans, the exact ancestry of the highly derived class Sagittozoa is still unknown. It comprises a couple hundred known species, all of them specialized parasites or predators, commonly known as "boltworms". The reason for the name is clear: their chitinous rostrum extends like an arrowhead from their slug-like body, often enriched with hooks or scales. The body of a boltworm is soft, flat, covered in mucus, usually camouflaged by reddish-brown streaks and spots, and, except for the rostrum, devoid of any hard part.
Most of them are very small, and live as bloodsucking parasites embedded in the skin of the largest animals (a polyphant killed in 1875 was reported to have over 200 boltworms of three different species on its back). Other have remained, or perhaps returned, in water: the swimming dartleech or knifeleech (Machairobdella oxyops), albeit tiny, is a dreaded presence in the streams of all the northern hemisphere.
The order Trigoniamorpha, or "flechettes", are predator boltworms with a body shaped like an isosceles triangle, and despite their size, larger than the parasites but still rather small, can be a grave danger even for humans. Their predation technique is strenuous but effective: having reached the high branches of a cathedral tree or similar arboreal plants, they detach and let themselves fall for a hundred feet or more. Thanks to their streamlined shape and the front heavier than the back, they fall with their rostrum pointed downwards, and they strike the flesh of their prey with deadly results.
The barbed rostrum easily burrows through the flesh as the flechette engorges itself. Even large preys die quickly from blood loss and infection; these creatures are wasteful predators, and as soon as their prey dies (or when they fall outside of the body) they leave it to crawl up a new tree. The climb is slow, and often lasts for days; their flat, slimy, unconspicuous body help them to ascend unnoticed. Most flechettes are one or two inches long, but the largest one, the goliath arrowtooth (Phobodus delabens) can be over eight inches long, almost half of which are taken by its unbelievably sharp rostrum.
More than a human life have been lost to these fearsome, unmerciful predators, lacking even the savage grace of the leaping cutters; yet, they still can elicit wonder with the peculiar details of their natural history. How did they evolve such a formidable weapon? How can they sense their prey's position at a distance when they bear no sign of eyes or hearing organs? Where do they fit in the intricate tree of horoid life?»
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Tarturus's avatar
Interesting concept here, how the worms use their spiky rostra for different purposes.