THE SPERMATOZOON OF BRACHIONUS-PLICATILIS (ROTIFERA, MONOGONONTA) WITH SOME NOTES ON SPERM ULTRASTRUCTURE IN ROTIFERA

The spermatozoon of B. plicatilis is a thread-like cell with an anterior flagellar portion and a posterior cell body. The flagellum has a lateral 'undulating membrane', containing a folded longitudinal cisterna and an axoneme. The basal body of the axoneme is at the anterior tip. The axoneme lacks outer dynein arms and extends through the entire flagellar region and most of the cell body. The main portion of the flagellum and of the cell body contains a series of vesicles with tightly packed tubules that may serve as a cytoskeleton. The cell body contains a partly condensed nucleus, several mitochondria and some cytoplasm. Some elongated mitochondria are arranged in the postnuclear region. When the spermatozoon moves, the undulations propagate from the basal body at the flagellar tip. Late spermatids can be recognized by the nucleus and the flagellum being coiled and enclosed within a common cell membrane. As in other rotifers, there are cigar-like cell products ('rods') in the testes. The general organization of the cell, including the absence of an evident acrosome, resembles that of the other known monogonont sperm types.