During reciprocal copulation inArchidoris pseudoargusseminal fluid is pumped into the bursa copulatrix of the partner and spermatozoa then pass to the receptaculum seminis where they are nourished and stored. The breakdown of excess gametes in the bursa and the relationship between the receptacular endothelium and the stored allosperms (i.e. foreign sperms received during copulation) were investigated with the electron microscope. Intrinsic allosperm motility probably plays no part in the translocation of semen either in the male or the female tracts. When oviposition occurs ripe oocytes are conducted along a ciliated tract through dense masses of autosperms (the individual’s own spermatozoa) in the vesicula seminalis. They meet active allosperms in the capsule-gland, and fertilization occurs. Many valve-mechanisms were discovered which guide the gametes during various reproductive activities. Female gametes are transported solely by ciliary means, whereas male gametes are translocated almost entirely by peristaltic muscular activities of the ducts. The axis of the spermatozoan tail is formed of a fibre-bundle, around which a prominent keel spirals to the tip. During flagellation in normal allosperms, uniplanar, bilaterally symmetrical waves originate in the neck and progress rearwards along the tail. In forward progression allosperms spin in a clockwise direction when viewed from the front. The suggestion is advanced that spinning is connected with differential alterations of the moving spermatozoon’s resistance to torque, brought about by the spiral keel, and the helical shape of the head itself; experimental support is furnished. A normal table of embryonic development was compiled for cultures maintained at 10 °C. Hatching occurs on the 28th day. The veligers are liberated at an earlier stage of development than those ofAdalaria proxima,andTritonia hombergi(Thompson 1958a, 1962). Field observations onArchidorisin a variety of localities yielded a confusing picture when pooled. When the population on a single stretch of shore was studied more closely there was no doubt that the life cycle was an annual one, and the maximal life span was approximately one year. Young individuals came into the samples in early autumn. They grew rapidly and gametogenesis began. Within a month some oocytes had ripened to apparent maturity and the production of tailed spermatozoa was under way. During the winter months more and more oocytes were brought to maturity. Ripe autosperms passed to the vesicula seminalis. Adults reached their maximal size in the spring, when spawning began; copulation and oviposition took place time and again. Feeding declined, but, so long as food reserves remained, new waves of ripening oocytes replenished the gonad. Finally, death occurred when the digestive gland was so shrunken that the digestive cells rounded off and drifted away from the basement membrane of the lobules. The life cycle varies slightly in different areas around the coast of Britain. A second, autumnal breeding season may occur in some localities. There is no reliable evidence that the maximal life span anywhere greatly exceeds one year.