134
looks like a narrow slit-like lumen (Figs. 2.15A, B , 2.16 ,
2.20 , and 2.22A ). Further calcifi cation results in partial
merging of the ento- and ectooecium (Fig. 2.20 ). The ooecial coelom transforms into a network of fl at anastomosing
lacunae connecting the coelom of the ooecial roof with the
visceral coelom of the distal zooid, sometimes disappearing
completely as in Corbulella maderensis (Fig. 2.22B ).
Similarly, the arched communication slit formed early in the
course of ovicellogenesis is gradually reduced to become
small communication pores (Figs. 2.20 and 2.21 ), usually
plugged by non-specialized epithelial cells. In the deepwater taxa Bryocalyx and Concertina , calcifi cation is very
weak, and the structure of the ooecium does not appear to
change with age (Ostrovsky and Schäfer 2003 ; Ostrovsky
et al. 2009a ).
As a rule, the communication pore(s) is plugged with
non-specialized epithelial cells (Figs. 2.15B , 2.16 , and
2.22A ), and it appears that coelomic fl uid is unable to circulate freely between the ooecium and distal zooid.
Nevertheless, the groups of epithelial cells that have been
seen at the base of the ooecial fold in sections of the developing ooecium do not plug the entire slit-like entrance to its
cavity. Moreover, in C . lineata , two complete ovicells with
embryos were found whose communication pores were also
free of cells (at least partially) (Fig. 2.15A ). Thus, in both
cases, coelomic fl uid should freely circulate between the
cavity of the ooecium and that of the parent zooid. The discovery of ooecial folds with open communication and a lack
of specialized pore-cell complexes in the plugged communication pores together indicate that such ooecia are not kenozooids (see discussion in Sect. 2.1 ). As for ovicells with
communication pores plugged by epithelial cells, ongoing
calcifi cation of ooecial walls indicates that necessary substances are transported to their lining across epithelial cells
and intercellular spaces (Ostrovsky and Schäfer 2003 ).
The inner vesicle is a hollow non-calcifi ed evagination of
the distal wall of the maternal autozooid that closes the ovicell opening (Figs. 2.14B , 2.15A, B , 2.16 , 2.22 , and 2.23 ).
The cuticle of the vesicle wall facing the brood cavity is very
thin whereas that of the wall adjoining the fl attened area of
entooecium (ooecial edge surrounding the ovicell opening) in
Callopora and Tegella is thickened to form a “sclerite” (sic,
Santagata and Banta 1996 ). The outer sclerite surface forms
numerous tiny parallel “ribs,” presumably tightening the contact between the vesicle and the ooecial edge; such ribs are
sometimes also found at the surface of the vesicle proximal
wall. The sclerite bears a transverse crest (triangular in section) serving for attachment of the largest muscular bundle of
the ooecial vesicle (Figs. 2.15A, B , 2.16 , and 2.22A )
(Ostrovsky and Schäfer 2003 ; Ostrovsky et al. 2009a ).
The proximal (lower) ends of the muscle bundles that
effect contraction of the ooecial vesicle during larval release
are attached to the basal wall of the maternal autozooid (near
its intersection with the distal transverse wall)
or to the lower part of the transverse wall (Fig. 2.22A ).
In C . dumerilii , attachment may occur at both locations or
even at the intersection itself. The distal end of the largest
(upper) muscle bundle (presumably consisting of two broad
muscle bands) is attached to the sclerite (Figs. 2.15A, B and
2.16 ). The second group of muscles consists of several fi ne
bundles attached to the inner middle surface of the ooecial
vesicle wall (Figs. 2.15B and 2.22A ). The lower group of
very thin muscle strands is attached to the inner lower part of
the vesicle wall (Figs. 2.16A and 2.22A ). These data are preliminary and require checking with confocal laser microscopy. The distance between the attachment sites of the
middle and the lower groups of muscles varies depending on
the ovicell. In C . dumerilii these two groups
of muscles are sometimes attached to the wall surface in
the upper half of the vesicle. Compared to the parietal muscles of the frontal wall of the zooid, the muscle
bundles of the ooecial vesicle are much broader and have
larger attachment zones. Whereas Silén ( 1945 ) thought that
the ooecial vesicle of C . dumerilii contains only one muscle
bundle, Calvet’s ( 1900 , fi g. 45) fi ndings in confamilial
Amphiblestrum fl emingi (as Membranipora ) more or less
accord with my own results (Ostrovsky and Schäfer 2003 ;
Ostrovsky et al. 2009a ).
The cuticle of the ooecial vesicle is lined with fl at epidermal and peritoneal cells (Figs. 2.15A, B and 2.16 ). The latter
are connected by their projections to the cells of the funicular
cords that cross the vesicle cavity (Fig. 2.15A ). There is no
indication that these cells enlarge during incubation. A fi ne
layer of non-cellular substance was often present at the surface of the vesicle wall facing the brood cavity, especially in
the folds of the wall (Ostrovsky and Schäfer 2003 ).
The ooecial vesicle retains its shape by means of coelomic
pressure. Its elastic wall collapses readily during contraction
of its internal musculature. Larvae may exit the ovicell
whether or not the maternal zooid contains a functional
polypide. The musculature of the ooecial vesicle, being part
of the parietal muscular system, does not degenerate during
polypide recycling, a feature noted by Dyrynda and Ryland
( 1982 ) in Chartella papyracea that presumably also occurs
in other cheilostomes (Ostrovsky 1998 ). The mature larva
with its actively beating cilia rotates in the brood cavity,
leading to contraction of the muscles of the ooecial vesicle
and opening of the ovicell entrance (Silén 1945 ). Once the
larva leaves the brood chamber (Fig. 1.20D, E ), the vesicle
recovers and the ovicell entrance is closed. It may be conjectured that contraction of the muscle bundles of the ooecial
vesicle during larval release and their subsequent relaxation
are followed by contraction of the cystid parietal muscles,
resulting in redistribution of coelomic fl uid and recovery of
2 Cheilostome Brood Chambers: Structure, Formation, Evolution
looks like a narrow slit-like lumen (Figs. 2.15A, B , 2.16 ,
2.20 , and 2.22A ). Further calcifi cation results in partial
merging of the ento- and ectooecium (Fig. 2.20 ). The ooecial coelom transforms into a network of fl at anastomosing
lacunae connecting the coelom of the ooecial roof with the
visceral coelom of the distal zooid, sometimes disappearing
completely as in Corbulella maderensis (Fig. 2.22B ).
Similarly, the arched communication slit formed early in the
course of ovicellogenesis is gradually reduced to become
small communication pores (Figs. 2.20 and 2.21 ), usually
plugged by non-specialized epithelial cells. In the deepwater taxa Bryocalyx and Concertina , calcifi cation is very
weak, and the structure of the ooecium does not appear to
change with age (Ostrovsky and Schäfer 2003 ; Ostrovsky
et al. 2009a ).
As a rule, the communication pore(s) is plugged with
non-specialized epithelial cells (Figs. 2.15B , 2.16 , and
2.22A ), and it appears that coelomic fl uid is unable to circulate freely between the ooecium and distal zooid.
Nevertheless, the groups of epithelial cells that have been
seen at the base of the ooecial fold in sections of the developing ooecium do not plug the entire slit-like entrance to its
cavity. Moreover, in C . lineata , two complete ovicells with
embryos were found whose communication pores were also
free of cells (at least partially) (Fig. 2.15A ). Thus, in both
cases, coelomic fl uid should freely circulate between the
cavity of the ooecium and that of the parent zooid. The discovery of ooecial folds with open communication and a lack
of specialized pore-cell complexes in the plugged communication pores together indicate that such ooecia are not kenozooids (see discussion in Sect. 2.1 ). As for ovicells with
communication pores plugged by epithelial cells, ongoing
calcifi cation of ooecial walls indicates that necessary substances are transported to their lining across epithelial cells
and intercellular spaces (Ostrovsky and Schäfer 2003 ).
The inner vesicle is a hollow non-calcifi ed evagination of
the distal wall of the maternal autozooid that closes the ovicell opening (Figs. 2.14B , 2.15A, B , 2.16 , 2.22 , and 2.23 ).
The cuticle of the vesicle wall facing the brood cavity is very
thin whereas that of the wall adjoining the fl attened area of
entooecium (ooecial edge surrounding the ovicell opening) in
Callopora and Tegella is thickened to form a “sclerite” (sic,
Santagata and Banta 1996 ). The outer sclerite surface forms
numerous tiny parallel “ribs,” presumably tightening the contact between the vesicle and the ooecial edge; such ribs are
sometimes also found at the surface of the vesicle proximal
wall. The sclerite bears a transverse crest (triangular in section) serving for attachment of the largest muscular bundle of
the ooecial vesicle (Figs. 2.15A, B , 2.16 , and 2.22A )
(Ostrovsky and Schäfer 2003 ; Ostrovsky et al. 2009a ).
The proximal (lower) ends of the muscle bundles that
effect contraction of the ooecial vesicle during larval release
are attached to the basal wall of the maternal autozooid (near
its intersection with the distal transverse wall)
or to the lower part of the transverse wall (Fig. 2.22A ).
In C . dumerilii , attachment may occur at both locations or
even at the intersection itself. The distal end of the largest
(upper) muscle bundle (presumably consisting of two broad
muscle bands) is attached to the sclerite (Figs. 2.15A, B and
2.16 ). The second group of muscles consists of several fi ne
bundles attached to the inner middle surface of the ooecial
vesicle wall (Figs. 2.15B and 2.22A ). The lower group of
very thin muscle strands is attached to the inner lower part of
the vesicle wall (Figs. 2.16A and 2.22A ). These data are preliminary and require checking with confocal laser microscopy. The distance between the attachment sites of the
middle and the lower groups of muscles varies depending on
the ovicell. In C . dumerilii these two groups
of muscles are sometimes attached to the wall surface in
the upper half of the vesicle. Compared to the parietal muscles of the frontal wall of the zooid, the muscle
bundles of the ooecial vesicle are much broader and have
larger attachment zones. Whereas Silén ( 1945 ) thought that
the ooecial vesicle of C . dumerilii contains only one muscle
bundle, Calvet’s ( 1900 , fi g. 45) fi ndings in confamilial
Amphiblestrum fl emingi (as Membranipora ) more or less
accord with my own results (Ostrovsky and Schäfer 2003 ;
Ostrovsky et al. 2009a ).
The cuticle of the ooecial vesicle is lined with fl at epidermal and peritoneal cells (Figs. 2.15A, B and 2.16 ). The latter
are connected by their projections to the cells of the funicular
cords that cross the vesicle cavity (Fig. 2.15A ). There is no
indication that these cells enlarge during incubation. A fi ne
layer of non-cellular substance was often present at the surface of the vesicle wall facing the brood cavity, especially in
the folds of the wall (Ostrovsky and Schäfer 2003 ).
The ooecial vesicle retains its shape by means of coelomic
pressure. Its elastic wall collapses readily during contraction
of its internal musculature. Larvae may exit the ovicell
whether or not the maternal zooid contains a functional
polypide. The musculature of the ooecial vesicle, being part
of the parietal muscular system, does not degenerate during
polypide recycling, a feature noted by Dyrynda and Ryland
( 1982 ) in Chartella papyracea that presumably also occurs
in other cheilostomes (Ostrovsky 1998 ). The mature larva
with its actively beating cilia rotates in the brood cavity,
leading to contraction of the muscles of the ooecial vesicle
and opening of the ovicell entrance (Silén 1945 ). Once the
larva leaves the brood chamber (Fig. 1.20D, E ), the vesicle
recovers and the ovicell entrance is closed. It may be conjectured that contraction of the muscle bundles of the ooecial
vesicle during larval release and their subsequent relaxation
are followed by contraction of the cystid parietal muscles,
resulting in redistribution of coelomic fl uid and recovery of
2 Cheilostome Brood Chambers: Structure, Formation, Evolution