160
immersed ovicells. Later, endozooidal and immersed ovicells could serve as the basis for the evolution of internal
brood sacs. A fourth way is associated with overgrowth of
the ooecium by secondary calcifi cation. The effect is similar
– the brood cavity becomes immersed, in this case into the
frontal shield of the distal zooid.
Analysis of the literature and my own data indicate a
trend towards immersion of the brood cavity, accompanied
by reduction and, in some cases, the complete disappearance
of the ooecium. These changes occurred repeatedly within
the Cheilostomata (Ostrovsky and Taylor 2004 ). Levinsen
( 1909 , p. 72) and Harmer ( 1926 , р. 405) were the fi rst to note
that related, sometimes congeneric, species may exhibit both
well-developed ooecia and reduced ooecia or none at all.
Hastings ( 1964 , p. 250) also discussed the simultaneous
presence of hyperstomial and the “reduced and vestigial ovicells” within the same cheilostome genera (see also Cook
1968a ). It is important to note that, while Harmer ( 1926 ,
p. 202) wrote concerning “the entozooecial ovicell … to
have preceded the hyperstomial ovicell in evolution, and to
have given rise to it”, on page 405 he described “forms with
well developed ovicells, which are in a course of reduction in
this genus, as has probably occurred in other lineages of
cheilostome evolution”.
Among the bryozoan groups that I have studied, this trend
is most prominent in the Calloporidae, in which hyperstomial
( Wilbertopora , Gilbertopora , Callopora , Tegella , Corbulella ,
Concertina , Bryocalyx , Amphiblestrum ) and subimmersed
( Valdemunitella ) ovicells with well-developed ooecia are
found alongside immersed ovicells with vestigial ooecia
( Crassimarginatella ) and internal brood sacs with vestigial
kenozooidal ooecia ( Cauloramphus ) (Ostrovsky et al. 2007 ,
2009a ). Gontarella , characterized by internal brood sacs and
no ooecium, may also belong to this family (Ostrovsky et al.
2006 ). It should be noted that the more the brood cavity is
immersed and the ooecium reduced in Calloporidae, the
smaller is the gymnocyst of the ovicell fl oor.
Endozooidal ovicells in Flustridae appear to have
resulted from a change in the growth processes at the edge
of the developing ooecial fold. Two descriptions of ovicellogenesis in fl ustrids are those of Vigelius ( 1884a , p. 50,
non-numbered text-fi g.) and Levinsen ( 1909 , pp. 57–58,
pl. 19, fi g. 8b-n). According to the former author the formation of the ovicell fl oor and brood cavity is because of
the invagination of the proximal part of the frontal wall of
the distal zooid. According to Levinsen ( 1909 ) it is the distal wall of the maternal zooid that invaginates. In both
descriptions, however, formation of the ovicell is accompanied by the growth and curvature of the transverse wall
between the maternal and daughter zooids. One may
suggest that such ovicellogenesis might involve activity of
the intercalary growth zone formed on the margin of the
ooecial fold. If so, the newly formed parts of the originally
non-calcifi ed entooecium should become immersed and
not raised as in hyperstomial ovicells. Additional studies
of ovicellogenesis in fl ustrids are necessary to determine
the details of this process.
Different expressions of ooecium reduction and the corresponding immersion of the brood cavity are found in many
cheilostome families. Both hyperstomial and immersed
ovicells may be present within the same genus ( Bugula ,
Camptoplites ) (Robertson 1905 ; Harmer 1926 ; Osburn 1950 ;
Bobin and Prenant 1963 ; Prenant and Bobin 1966 ; Ryland
and Hayward 1977 , 1992 ; Gordon 1986 ; Hayward 1995 ;
Soule et al. 1995 ), and the same genus may contain some
species with immersed ovicells and others with internal
brood sacs ( Himantozoum , Caulibugula ) (Harmer 1926 ;
Hastings 1943 , 1945 , 1964 ; Gordon 1986 ; Hayward 1995 ).
The same trend is found in Farciminellum (Farciminariidae),
Menipea (Candidae) and Beania (Beaniidae), which include
some species with well-developed ooecia, others with vestigial oecia, and some with none at all (i.e. with internal brood
sacs) (Harmer 1926 ; Hastings 1943 ; Osburn 1950 ; Gordon
1984 , 1986 ; Zabala and Maluquer 1988 ; Hayward 1995 ;
Ostrovsky, unpublished data).
The trend towards immersion of the brood cavity and
reduction of the ooecium is also observed in Recent species
of Cellaria (Cellariidae) (see illustrations in Hayward 1995 ;
Ostrovsky, unpublished data). Judging from illustrations published by Cook and Chimonides ( 1985 , 1986 , 1987 ), Cadée
et al. ( 1989 ), Parker and Cook ( 1994 ), Håkansson and Voigt
( 1996 ), and Bock and Cook ( 1999 ), species of Lunularia
(Lunulariidae), Pseudolunularia and Selenaria (Selenariidae),
and Lunulites and Pavolunulites (Lunulitidae) have ovicells
with a vestigial ooecium and brood sac immersed into the
cavity of the maternal autozooid. The vestigial ooecium may
be developed to varying degrees – it is sometimes quite
distinct but more often barely discernible. In Setosellina
(Heliodomidae), ooecia may be present or absent (Harmer
1926 ; Harmelin 1977 ).
A similar trend is found at family level (Ostrovsky
et al. 2006 , 2009a ; see also Table 2.1 ). Most genera in
the following families have ovicells, exceptions being
Oshurkovia (Umbonulidae) (Hastings 1944 , 1964 ;
Eggleston 1972 ; Grischenko and Mawatari 2005 ),
Arctonula (Romancheinidae) (Kluge 1975 ; Gordon and
Grischenko 1994 ; Hayward and Ryland 1999 ), Fatkullina
(Stomachetosellidae) (Grischenko et al. 1998 ) and
Odontoporella (Hippoporidridae) (Canu and Bassler
1929 ; Osburn 1950 ; Prenant and Bobin 1966 ; Ryland and
Hayward 1977 ; Gordon 1989a ; Hayward 1995 ), with
internal brooding. Actual brood sacs have been demon2 Cheilostome Brood Chambers: Structure, Formation, Evolution
immersed ovicells. Later, endozooidal and immersed ovicells could serve as the basis for the evolution of internal
brood sacs. A fourth way is associated with overgrowth of
the ooecium by secondary calcifi cation. The effect is similar
– the brood cavity becomes immersed, in this case into the
frontal shield of the distal zooid.
Analysis of the literature and my own data indicate a
trend towards immersion of the brood cavity, accompanied
by reduction and, in some cases, the complete disappearance
of the ooecium. These changes occurred repeatedly within
the Cheilostomata (Ostrovsky and Taylor 2004 ). Levinsen
( 1909 , p. 72) and Harmer ( 1926 , р. 405) were the fi rst to note
that related, sometimes congeneric, species may exhibit both
well-developed ooecia and reduced ooecia or none at all.
Hastings ( 1964 , p. 250) also discussed the simultaneous
presence of hyperstomial and the “reduced and vestigial ovicells” within the same cheilostome genera (see also Cook
1968a ). It is important to note that, while Harmer ( 1926 ,
p. 202) wrote concerning “the entozooecial ovicell … to
have preceded the hyperstomial ovicell in evolution, and to
have given rise to it”, on page 405 he described “forms with
well developed ovicells, which are in a course of reduction in
this genus, as has probably occurred in other lineages of
cheilostome evolution”.
Among the bryozoan groups that I have studied, this trend
is most prominent in the Calloporidae, in which hyperstomial
( Wilbertopora , Gilbertopora , Callopora , Tegella , Corbulella ,
Concertina , Bryocalyx , Amphiblestrum ) and subimmersed
( Valdemunitella ) ovicells with well-developed ooecia are
found alongside immersed ovicells with vestigial ooecia
( Crassimarginatella ) and internal brood sacs with vestigial
kenozooidal ooecia ( Cauloramphus ) (Ostrovsky et al. 2007 ,
2009a ). Gontarella , characterized by internal brood sacs and
no ooecium, may also belong to this family (Ostrovsky et al.
2006 ). It should be noted that the more the brood cavity is
immersed and the ooecium reduced in Calloporidae, the
smaller is the gymnocyst of the ovicell fl oor.
Endozooidal ovicells in Flustridae appear to have
resulted from a change in the growth processes at the edge
of the developing ooecial fold. Two descriptions of ovicellogenesis in fl ustrids are those of Vigelius ( 1884a , p. 50,
non-numbered text-fi g.) and Levinsen ( 1909 , pp. 57–58,
pl. 19, fi g. 8b-n). According to the former author the formation of the ovicell fl oor and brood cavity is because of
the invagination of the proximal part of the frontal wall of
the distal zooid. According to Levinsen ( 1909 ) it is the distal wall of the maternal zooid that invaginates. In both
descriptions, however, formation of the ovicell is accompanied by the growth and curvature of the transverse wall
between the maternal and daughter zooids. One may
suggest that such ovicellogenesis might involve activity of
the intercalary growth zone formed on the margin of the
ooecial fold. If so, the newly formed parts of the originally
non-calcifi ed entooecium should become immersed and
not raised as in hyperstomial ovicells. Additional studies
of ovicellogenesis in fl ustrids are necessary to determine
the details of this process.
Different expressions of ooecium reduction and the corresponding immersion of the brood cavity are found in many
cheilostome families. Both hyperstomial and immersed
ovicells may be present within the same genus ( Bugula ,
Camptoplites ) (Robertson 1905 ; Harmer 1926 ; Osburn 1950 ;
Bobin and Prenant 1963 ; Prenant and Bobin 1966 ; Ryland
and Hayward 1977 , 1992 ; Gordon 1986 ; Hayward 1995 ;
Soule et al. 1995 ), and the same genus may contain some
species with immersed ovicells and others with internal
brood sacs ( Himantozoum , Caulibugula ) (Harmer 1926 ;
Hastings 1943 , 1945 , 1964 ; Gordon 1986 ; Hayward 1995 ).
The same trend is found in Farciminellum (Farciminariidae),
Menipea (Candidae) and Beania (Beaniidae), which include
some species with well-developed ooecia, others with vestigial oecia, and some with none at all (i.e. with internal brood
sacs) (Harmer 1926 ; Hastings 1943 ; Osburn 1950 ; Gordon
1984 , 1986 ; Zabala and Maluquer 1988 ; Hayward 1995 ;
Ostrovsky, unpublished data).
The trend towards immersion of the brood cavity and
reduction of the ooecium is also observed in Recent species
of Cellaria (Cellariidae) (see illustrations in Hayward 1995 ;
Ostrovsky, unpublished data). Judging from illustrations published by Cook and Chimonides ( 1985 , 1986 , 1987 ), Cadée
et al. ( 1989 ), Parker and Cook ( 1994 ), Håkansson and Voigt
( 1996 ), and Bock and Cook ( 1999 ), species of Lunularia
(Lunulariidae), Pseudolunularia and Selenaria (Selenariidae),
and Lunulites and Pavolunulites (Lunulitidae) have ovicells
with a vestigial ooecium and brood sac immersed into the
cavity of the maternal autozooid. The vestigial ooecium may
be developed to varying degrees – it is sometimes quite
distinct but more often barely discernible. In Setosellina
(Heliodomidae), ooecia may be present or absent (Harmer
1926 ; Harmelin 1977 ).
A similar trend is found at family level (Ostrovsky
et al. 2006 , 2009a ; see also Table 2.1 ). Most genera in
the following families have ovicells, exceptions being
Oshurkovia (Umbonulidae) (Hastings 1944 , 1964 ;
Eggleston 1972 ; Grischenko and Mawatari 2005 ),
Arctonula (Romancheinidae) (Kluge 1975 ; Gordon and
Grischenko 1994 ; Hayward and Ryland 1999 ), Fatkullina
(Stomachetosellidae) (Grischenko et al. 1998 ) and
Odontoporella (Hippoporidridae) (Canu and Bassler
1929 ; Osburn 1950 ; Prenant and Bobin 1966 ; Ryland and
Hayward 1977 ; Gordon 1989a ; Hayward 1995 ), with
internal brooding. Actual brood sacs have been demon2 Cheilostome Brood Chambers: Structure, Formation, Evolution