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'.