137
and peritoneal cells. The position of the ooecium does not
prevent distal budding of the maternal autozooid.
The brood cavity is immersed in the distal part of the
maternal autozooid and looks like a spacious sac with thin
non-calcifi ed walls. It consists of a main chamber and a fl at
neck leading to the exterior. The entrance to the brood cavity
is tightly closed by a specialized part of the distal wall of the
maternal autozooid functioning as an ooecial vesicle. When
it is displaced, the brood cavity communicates directly with
the outside world and not with the vestibulum. At the site
where the ooecial vesicle tightly adjoins the entooecial surface, its wall has a cuticular thickening that appears to be a
homologue of the sclerite in other calloporids (Fig. 2.25B ).
A group of muscles (possibly paired) that ensure displacement of the fold and opening of the brood chamber during
oviposition and larval release is attached to the wall of the
fold above and below the sclerite. At their opposite ends,
these muscles are presumably attached to the lateral walls
of the cystid. Groups of muscles are also attached to the
neck and main chamber of the brood sac (Ostrovsky et al.
2007 , 2009a ).
A vestigial kenozooidal ooecium is formed in Cymulopora
uniserialis (see Winston and Håkansson 1986 ), but the structure of the brood chamber in this species remains unknown.
Thus, the family Calloporidae (sensu lato) has a diverse
range of brood chambers, indicating the existence of several
trends in the evolution of this earliest group of brooding
cheilostomes. These trends include reduction of the distal
ooecium-bearing zooid, immersion of the brood cavity
accompanied by its proximal displacement and reduction in
ooecium size, as well as closure of the ovicell opening by the
zooidal operculum (transition from acleithral to cleithral
type). Recently, it has been suggested that Gontarella , with
internal brooding and lacking an ooecium, belongs to the
Calloporidae (see Ostrovsky et al. 2009b ), in which case calloporids span the entire morphological series from external
ovicells to internal incubation.
2.3.2 Structure and Development of Ovicells
in Other Cheilostome Families
Apart from ovicells with ooecia constructed from spines
and costae (see Sects. 2.3.1 , 2.4.3 , 2.4.4 , 2.4.5 , and 2.4.6 ),
there are at least fi ve other variants of ooecium structure in
cheilostome brooders, all of them modifi cations of the basic
calloporid plan known since the Cenomanian. The main
criteria used for delimiting these variants are (1) the mode of
ooecial- wall calcifi cation, (2) degree and mode of contact of
ooecial walls with the skeletal elements of the frontal wall/
shield of the distal zooid, (3) mode of communication
between the ooecial coelom and the zooidal (visceral or
hypostegal) coelom, and (4) details of ovicellogenesis. All of
these characters are subject to variation within the ‘frame’ of
the particular variant, whereas in some species ovicell structure combines characters of different variants. Moreover,
variability characterizes the early stages of ooecial-fold
formation, methods of ovicell closure, degree of immersion
of the brood cavity, structure of the ooecial vesicle including
shape and size, degree of sclerite development, number of
muscular bundles and the loci of their attachment as well as
some other characters. In fact, structural variability is so
great that one can present only a brief comparative analysis
of ovicell diversity across the major cheilostome clades.
In order to do this, it is convenient to refer to the major variants as “calloporiform,” “escharelliform,” “lepralielliform”
and “microporelliform” in the account that follows.
2.3.2.1 The Calloporiform Ooecium
Despite the vast structural diversity, ooecial morphology in
most studied cheilostomes conforms to the calloporiform
type (see Fig. 1 in Introduction, Figs. 2.15A, B , 2.16 , and
2.22 ). This type of ooecium is a double-walled hemispheric
outgrowth with a completely calcifi ed entooecium, a completely or partly calcifi ed ectooecium and a slit-like coelomic
cavity between them. The ooecial coelom communicates
with the zooidal coelomic cavity via an arched slit or pores
derived from it, which may be open or plugged by non-specialized epithelial cells, or via communication pore(s) with a
pore- cell complex. Apart from calloporids (Figs. 2.6a (A, B, E),
2.7a (C, F), b (B), 2.8B , 2.11A , 2.12D, E , 2.13 , 2.14 , 2.15 ,
2.16 , 2.17 , 2.18 , 2.19 , 2.20 , 2.21 , 2.22 , 2.23 , 2.24 , and 2.25A ),
such an ooecium – whether well-developed or vestigial,
complete or bilobate (with lobes fused to varying degrees),
an outgrowth of the distal zooid or a kenozooid budded from
the maternal autozooid – is characteristic of (1) the anascan
fl ustrine superfamilies Calloporoidea (e.g. families
Chaperiidae, Hiantoporidae, Farciminariidae) (Fig. 2.6b (C)),
Flustroidea (Flustridae, except for species with internal
brood sacs lacking an ooecium) (Figs. 1.17 , 2.7b (A), 2.31 ,
and 2.32 ), Buguloidea (Candidae, some Bugulidae)
(Figs. 2.6a (F), 2.7a (B), 2.8A, C , and 2.30 ), and Microporoidea
(some Microporidae) (Figs. 2.33F and 2.63A–C ); (2) the
acanthostegan families Cribrilinidae, Euthyroididae,
Bifaxariidae, Catenicellidae, and Eurystomellidae (Figs.
1.24A , 1.25A , 1.32A, B , 2.6a (D), b (A, B), 2.7a (H, I), 2.27 ,
2.28 , and 2.29 ); (3) the gymnocystal-shielded ascophoran
family Hippothoidae (Figs. 1.27D , 1.30B , and 1.36 ); (4)
some members the umbonuloid-shielded family
Arachnopusiidae; and (5) some members of the
“lepraliomorph” family Smittinidae (at least two species)
(Vigelius 1884a , b ; Calvet 1900 ; Levinsen 1909 ; Woollacott
and Zimmer 1972a ; Wass and Banta 1981 ; Nielsen 1985 ;
Lobastova and Ostrovsky 1994 ; Santagata and Banta 1996 ;
Ostrovsky 1998 , 2002 ; Ostrovsky and Schäfer 2003 ;
Ostrovsky et al. 2003 , 2009a , unpublished data). The base of
2.3 Structure and Development of Brood Chambers in Cheilostomata
and peritoneal cells. The position of the ooecium does not
prevent distal budding of the maternal autozooid.
The brood cavity is immersed in the distal part of the
maternal autozooid and looks like a spacious sac with thin
non-calcifi ed walls. It consists of a main chamber and a fl at
neck leading to the exterior. The entrance to the brood cavity
is tightly closed by a specialized part of the distal wall of the
maternal autozooid functioning as an ooecial vesicle. When
it is displaced, the brood cavity communicates directly with
the outside world and not with the vestibulum. At the site
where the ooecial vesicle tightly adjoins the entooecial surface, its wall has a cuticular thickening that appears to be a
homologue of the sclerite in other calloporids (Fig. 2.25B ).
A group of muscles (possibly paired) that ensure displacement of the fold and opening of the brood chamber during
oviposition and larval release is attached to the wall of the
fold above and below the sclerite. At their opposite ends,
these muscles are presumably attached to the lateral walls
of the cystid. Groups of muscles are also attached to the
neck and main chamber of the brood sac (Ostrovsky et al.
2007 , 2009a ).
A vestigial kenozooidal ooecium is formed in Cymulopora
uniserialis (see Winston and Håkansson 1986 ), but the structure of the brood chamber in this species remains unknown.
Thus, the family Calloporidae (sensu lato) has a diverse
range of brood chambers, indicating the existence of several
trends in the evolution of this earliest group of brooding
cheilostomes. These trends include reduction of the distal
ooecium-bearing zooid, immersion of the brood cavity
accompanied by its proximal displacement and reduction in
ooecium size, as well as closure of the ovicell opening by the
zooidal operculum (transition from acleithral to cleithral
type). Recently, it has been suggested that Gontarella , with
internal brooding and lacking an ooecium, belongs to the
Calloporidae (see Ostrovsky et al. 2009b ), in which case calloporids span the entire morphological series from external
ovicells to internal incubation.
2.3.2 Structure and Development of Ovicells
in Other Cheilostome Families
Apart from ovicells with ooecia constructed from spines
and costae (see Sects. 2.3.1 , 2.4.3 , 2.4.4 , 2.4.5 , and 2.4.6 ),
there are at least fi ve other variants of ooecium structure in
cheilostome brooders, all of them modifi cations of the basic
calloporid plan known since the Cenomanian. The main
criteria used for delimiting these variants are (1) the mode of
ooecial- wall calcifi cation, (2) degree and mode of contact of
ooecial walls with the skeletal elements of the frontal wall/
shield of the distal zooid, (3) mode of communication
between the ooecial coelom and the zooidal (visceral or
hypostegal) coelom, and (4) details of ovicellogenesis. All of
these characters are subject to variation within the ‘frame’ of
the particular variant, whereas in some species ovicell structure combines characters of different variants. Moreover,
variability characterizes the early stages of ooecial-fold
formation, methods of ovicell closure, degree of immersion
of the brood cavity, structure of the ooecial vesicle including
shape and size, degree of sclerite development, number of
muscular bundles and the loci of their attachment as well as
some other characters. In fact, structural variability is so
great that one can present only a brief comparative analysis
of ovicell diversity across the major cheilostome clades.
In order to do this, it is convenient to refer to the major variants as “calloporiform,” “escharelliform,” “lepralielliform”
and “microporelliform” in the account that follows.
2.3.2.1 The Calloporiform Ooecium
Despite the vast structural diversity, ooecial morphology in
most studied cheilostomes conforms to the calloporiform
type (see Fig. 1 in Introduction, Figs. 2.15A, B , 2.16 , and
2.22 ). This type of ooecium is a double-walled hemispheric
outgrowth with a completely calcifi ed entooecium, a completely or partly calcifi ed ectooecium and a slit-like coelomic
cavity between them. The ooecial coelom communicates
with the zooidal coelomic cavity via an arched slit or pores
derived from it, which may be open or plugged by non-specialized epithelial cells, or via communication pore(s) with a
pore- cell complex. Apart from calloporids (Figs. 2.6a (A, B, E),
2.7a (C, F), b (B), 2.8B , 2.11A , 2.12D, E , 2.13 , 2.14 , 2.15 ,
2.16 , 2.17 , 2.18 , 2.19 , 2.20 , 2.21 , 2.22 , 2.23 , 2.24 , and 2.25A ),
such an ooecium – whether well-developed or vestigial,
complete or bilobate (with lobes fused to varying degrees),
an outgrowth of the distal zooid or a kenozooid budded from
the maternal autozooid – is characteristic of (1) the anascan
fl ustrine superfamilies Calloporoidea (e.g. families
Chaperiidae, Hiantoporidae, Farciminariidae) (Fig. 2.6b (C)),
Flustroidea (Flustridae, except for species with internal
brood sacs lacking an ooecium) (Figs. 1.17 , 2.7b (A), 2.31 ,
and 2.32 ), Buguloidea (Candidae, some Bugulidae)
(Figs. 2.6a (F), 2.7a (B), 2.8A, C , and 2.30 ), and Microporoidea
(some Microporidae) (Figs. 2.33F and 2.63A–C ); (2) the
acanthostegan families Cribrilinidae, Euthyroididae,
Bifaxariidae, Catenicellidae, and Eurystomellidae (Figs.
1.24A , 1.25A , 1.32A, B , 2.6a (D), b (A, B), 2.7a (H, I), 2.27 ,
2.28 , and 2.29 ); (3) the gymnocystal-shielded ascophoran
family Hippothoidae (Figs. 1.27D , 1.30B , and 1.36 ); (4)
some members the umbonuloid-shielded family
Arachnopusiidae; and (5) some members of the
“lepraliomorph” family Smittinidae (at least two species)
(Vigelius 1884a , b ; Calvet 1900 ; Levinsen 1909 ; Woollacott
and Zimmer 1972a ; Wass and Banta 1981 ; Nielsen 1985 ;
Lobastova and Ostrovsky 1994 ; Santagata and Banta 1996 ;
Ostrovsky 1998 , 2002 ; Ostrovsky and Schäfer 2003 ;
Ostrovsky et al. 2003 , 2009a , unpublished data). The base of
2.3 Structure and Development of Brood Chambers in Cheilostomata