In this paper a new species of Syllid, named Pionosyllis neapolitana, is described, whose chief characteristics are that it is hermaphrodite, and has reproductive organs of remarkably complex and constant structure.1 There are a pair of testes in each of the segments 10, 11, and 12, and a pair of ovaries in every segment from the thirteenth backwards throughout the region of the true intestine. A pair of nephridia with small nephridiostomes occurs in every segment from the fifth backwards, except in segments 11, 12, and 13, in which they become transformed into nephromixia functioning as sperm-ducts. Each sperm-duct is provided with a ciliated coelomostome opening into a male segment, and its postseptal tubule is enlarged into a sperm-sac where the spermatozoa form spermatophores. Presumably copulation takes place, since spermatophores are found lodged in paired spermathecae opening to the exterior on every female segment. One ovum at a time in every ovary enlarges and is extruded dorsally, apparently by breaking through the body-wall. The ova by this time are fertilized. They develop to an advanced stage surrounded by a cuticular membrane, and attached in pairs to every female segment. The young escape from the membrane when about eighteen segments have been formed. When the ova pass to the exterior they become attached to the laterodorsal surface of the female segments by means of fixing threads formed by special paired organs of fixation. These organs are derived from the spermathecae. Possibly successive generations of ova are extruded, but this has not yet been observed, nor is it known whether the fixing organs can again function as spermathecae. Exactly how and when fertilization takes place has not so far been determined.

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