The life history ofSyndisyrinx franciscanus, a symbiotic turbellarian from the intestine of echinoids, with observations on the mechanism of hatching

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Abstract
The life history of the umagillid flatworm, Syndisyrinx franciscanus, was studied using specimens inhabiting the intestine of the sea urchins Strongylocentrotus pallidus and S. droebachiensis. Egg capsules contain 2–8 oocytes and several hundred yolk cells. The capsules are produced at a maximum rate of about 1 per 1 ½ days; they are released one at a time into the intestine of the host and pass to the outside with feces. Embryogenesis lasts approximately two months. Fully formed embryos can survive in the egg capsules in a semi-dormant state for 10–11 additional months. They hatch when the capsules are ingested by an urchin. Hatching involves an apparent shattering of part of the egg capsule wall. Experiments revealed that hatching is induced by some property of the intestinal fluids of the urchins, and that the changes in the capsule wall are mediated by the embryos, perhaps by the secretion of a hatching enzyme. Recently emerged juveniles possess an anterior gland that is unique to this stage of the life history. The worms mature within the intestine of a single host.