The
spermatozoa of two genera and
species of Cyclodorippidea, Xeinostoma
richeri (Xeinostominae) and Tymolus
sp. (Cyclodorippinae), and one species
of Cymonomus
sp. (Cymonomidae) are found to constitute a distinctive
cyclodorippoid
sperm-type characterized by (1) sperm anteroposteriorly
depressed, mean
ratio of length to width 0.54-0.63; (2) operculum
extending to the
lateral limits of the acrosome (autapomorphy of
cyclodorippoids)
and centrally perforate or (Cymonomus sp.) thinner; (3)
contents of
acrosome vesicle with two major horizontal zones, as in
homolids and
dynomenids, including a dense lower (posterior) zone; (4)
perforatorium very
wide (0.3 width of acrosome), anteriorly rounded, not
capitate, lacking
radiate projections; (5) acrosomal capsule with external
projections over
its posterior half, (6) slender dense filaments extending
into the
perforatorium from its walls, their bases associated with
corrugations of
its basal wall; (7) nucleus, cupping the acrosome and
cytoplasm, with
well-developed posterior median process; (8) nuclear arms
lacking
microtubules; (9) cytoplasm, a narrow postacrosomal band extending
anteriorly as far
as the operculum, associated with a few degenerate
mitochondria. The
noncapitate form of the perforatorium differs from the
capitate condition
in dromiids, the related dynomenids, homolids and the
raninoid Lyreidus.
The cyclodorippoid sperm resembles homolid and raninoid
sperm in
possessing a posterior nuclear process (questionably apomorphic)
and resembles
homolid sperm in the horizontal zonation of the acrosome
with a dense lower
zone. Features which resemble the sperm of raninoids
are: the
periacrosomal flange (Xeinostoma richeri) and smaller
evaginations of
the acrosome membrane (or capsule?) (X. richeri, less
distinctly Cymonomus
sp. and Tymolus sp.) reminiscent of the single
acrosomal flange
of Ranina and Raninoides sperm and the multiple keels of
the Lyreidus
sperm; and corrugations of the wall of the perforatorial
chamber, as in
raninoids though with significant differences. A dynomenid
similarity
(homoplasy?) is the discontinuous flange-like peripheral
continuation of
the lower zone of the acrosome contents in Cymonomus sp.