Pristina leidyi are tiny, delicate, largely transparent annelid worms. "Individuals are typically 0.15-0.20 mm in diameter, 2-4 mm in length, and comprised of ~15-35 segments. An asegmental cap of tissue is present at either end of the worm, the posterior cap being the pygidium, and the anterior cap being the prostomium (elongated into a proboscis in this species). A proliferative region just anterior to the pygidium, the posterior growth zone, produces new segments during normal growth. Fission zones are typically formed between segments 15 and 18." (Zattara and Bely in press)
"Segments have a muscular body wall, composed of a single-layered epidermis underlain by longitudinal and circular muscles, and a spacious coelomic cavity subdivided by septa into segmentally iterated pouches. In each segment, four bundles of chaetae protrude from the body wall, a left/right pair of ventrolateral bundles and a left/right pair of dorsolateral bundles" (Zattara and Bely in press). Ventral chaetae are hooked and relatively short, while dorsal chaetae are long and hair like. The dorsal chaetae of the second segment are much longer than the rest. "The digestive tract is comprised of an anteroventral mouth and buccal cavity which are unciliated followed by a protrusible pharynx, esophagus, stomach (in segment 7), and hindgut, all of which are ciliated. In segments posterior to segment 7, nephridia (ciliated excretory structures) are typically present laterally, beneath the body wall, on one or both sides of the segment. A dorsal cerebral ganglion lies above the foregut in segment 1 and a ganglionated ventral nerve cord runs along the length of the body, with one ganglion per segment. A nerve loop connects the cerebral ganglion and ventral nerve cord: a left/right pair of circumenteric connectives extends anterodorsally from the ventral nerve cord ganglion of segment 1, loops around the foregut, and connects in the cerebral ganglion, forming the cerebral commissure connecting the left and right halves of the brain. Three subepidermal peripheral nerve rings (nr1, nr2, and nr3, from anterior to posterior) occur in all segments posterior to segment 6. Ventrally each nerve ring connects to the ventral nerve cord, and fine extensions from these rings connect to epidermal sensory hair tufts. A main dorsal blood vessel and a main ventral blood vessel each run along the length of the animal, being connected to each other directly at the termini and by smaller transverse vessels along the length of the body" (Zattara and Bely in press).
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