ULTRASTRUCTURE OF THE SPERMATOZOON AND SPERMATOGENESIS OF THE BRACHIOPODS DISCINISCA-TENUIS (INARTICULATA) AND KRAUSSINA-RUBRA (ARTICULATA)

The structure of the spermatozoon and spermatogenesis of Discinisca te nuis (Inarticulata) and Kraussina rubra (Articulata) are described by transmission electron microscopy. The spermatozoon of D. tenuis has a similar morphology to that of other inarticulate brachiopods. The head (about 1.9 mum long) is composed of a nucleus which is invaginated an teriorly and posteriorly, the anterior invagination partially housing the pyriform acrosome. The acrosome is differentiated internally and i s separated from the nucleus by subacrosomal material. The mid-piece i s composed of a ring of four or five spherical mitochondria which surr ound the proximal and distal centrioles which are arranged orthogonall y. By contrast the spermatozoon of K. rubra has a head which is 2 mum long x 1.1 mum in diameter. The nucleus which is rounded anteriorly is capped by a small (0.5 mum diameter) disc-shaped acrosome. The mid-pi ece has a single ring-shaped mitochondrion which surrounds the centrio les which are almost in line with each other. In both species, during spermatogenesis, each spermatogonium and spermatocyte develops a flage llum which appears to be absorbed prior to cell division. In D. tenuis , proacrosomal vesicles form within spermatogonia whereas acrosome for mation in K. rubra does not commence until spermiogenesis. In the earl y spermatid of both species a single acrosomal vesicle forms in the pr esumptive posterior of the cell and during spermiogenesis the developi ng acrosome migrates anteriorly where it assumes its mature form.