Life history and function of the secondary uterus ofWahlia pulchella, an umagillid turbellarian from the intestine of a Northeastern pacific sea cucumber (Stichopus californicus)

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Abstract
The flatworm Wahlia pulchella is one of 15 members of the turbellarian family Umagillidae that possesses a secondary uterus, a dilation of the female tract in which egg capsules are stored. Egg capsules are formed in the primary uterus at rates of 1 every 2.5-5 h. They pass into the secondary uterus, where the capsules' walls harden for 12-24 h, and are then released. The time between formation and release of a single capsule is approximately the same for W. pulchella as for intestine-inhabiting umagillids that lack a secondary uterus and that, consequently, produce and release egg capsules one at a time. Because W. pulchella can form new capsules in the primary uterus before previously formed capsules have finished hardening, however, the rate of capsule production is 5-15 times greater than that of intestine-inhabiting species that lack a secondary uterus. Egg capsules pass out of the host with fecal material. Embryogenesis lasts about 60 days. Developed embryos hatch when capsules are ingested by the host, Stichopus califomicus; the life cycle is apparently direct. Wahlia pulchella disappears from S. califomicus during the fall and winter, when the viscera of the hosts are resorbed. The worms overwinter as embryos in egg capsules on the sea floor.