SEM was used to study the external morphology of spermatozoa from two species of Macrobiotus (Eutardigrada). In both species, the spermatozoa consists of 4 parts: (1) a red-shaped acrosome; (2) a helical nucleus; (3) an elongated, swollen middle piece, with lateral vesicles; (4) a tail ending in a tuft of thin and long filaments (probably microtubules). Observations made in vivo and on fresh preparations of sperm cells in toto confirm the images. Moreover, light microscopy findings also showed tahta spermatozoa in the testes and gonoducts have a folded-back acrosomal region, joined to the middle piece. Artificially extracted appeared in some cases straight-headed.
First SEM studies on tardigrade spermatozoa. Invertebr / Rebecchi, Lorena; A., Guidi. - In: INVERTEBRATE REPRODUCTION & DEVELOPMENT. - ISSN 0792-4259. - STAMPA. - 19:(1991), pp. 151-156.