The atypical spermatozoon of Theodoxus fluviatilis (L.) (Gastropoda, Prosobranchia)

The atypical spermatozoa of Theodoxus fluviatilis (L.) (Gastropoda, Prosobranchia, Neritoidea) can be divided into two classes of length even though they are identical in their internal structure. Each atypical spermatozoon is a monoflagellate cell completely lacking a nucleus. This would seem to result from an anomalous maturation of cells which in other respects resembles that giving rise to the typical line. The anomalous maturation becomes apparent at the spermatocyte stage. The final transformation of spermatid into spermatozoon is effected by an expulsion of the nucleus, enveloped in its own membrane, from the cytoplasm. The resulting mature atypical spermatozoon, is composed of a long flagellum encircled by a cytoplasmic sheath extending about one-seventh of its length. © 1983 Academic Press, Inc.