105. Jamieson, B.G.M. 1989b. The ultrastructure of the spermatozoa of four species of Xanthid crabs (Crustacea,  Brachyura, Xanthidae). Journal of Submicroscopic Cytology and Pathology 21, 579-584.

Features of xanthid sperm which are seen in other higher Brachyura and in
raninoids include: the large subspheroidal acrosome (a synapomorphy of the
raninoid + higher brachyuran assemblage contrasting with the disc-shaped

dromioid acrosome); enclosure of the acrosome by a thin layer of cytoplasm
which is in turn cupped by the nucleus; extension of the nucleus as
lateral arms; presence of cytoplasm (here vestigial) in the basal region
of each nuclear arm; absence of the posterior median process (presence
being a raninoid-majid symplesiomorphy), and topographical equivalence and
presumed homology of components of the acrosome, viz. the electron-dense
capsule; inner and outer dense zones, surrounding the longitudinal axis;
peripheral vesicular contents; an apical operculum; subopercular- or
subcap-zone; and basally open subacrosomal chamber enclosing perforatorial
material. Higher brachyuran characters of xanthids not seen in raninoids
include: anterior termination of the subacrosomal space and enclosed
perforatorium at the base of the operculum (contrasting with termination
at the equator of the acrosome in raninoids); modification of the capsule
around the base of the perforatorium as a thickened ring; absence of
longitudinal corrugations lining the subacrosomal chamber (presence is a
raninoid autapomorphy); and degeneration of all mitochondria (some
apparently persisting in raninoids). A xanthid autapomorphy is
differentiation of the posterior region of the inner dense zone
surrounding the perforatorium as a prominent strongly electron-dense ring,
the 'xanthid ring'.