n.1.(Zool.) Any one of very numerous species of small spiral gastropods of the genus Rissoa, or family Rissoidæ, found both in fresh and salt water.
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Two of the Truncatella species have an accessory sperm storage organ [ILLUSTRATION FOR FIGURE 5C-D OMITTED]; it resembles the upper oviduct gland in rissoids, which in some taxa has sperm-storing lobes (Ponder 1985a).