130
“Peristomial” ovicells (Levinsen 1902 , 1909 ), in which the
ooecial capsule is incorporated into the zooidal peristome,
comprise subimmersed or sometimes endozooidal types
(Figs. 2.7a (D, E) and 2.37 ).
Further immersion of the brood cavity in the maternal
zooid, concurrent with reduction of the ooecial fold, eventually results in “internal sacs,” “internal embryo sacs,” or
“membranous diverticula” (Waters 1912 ; Ström 1977 ; Cook
1979 ), in which the cavity can be connected with that of
the introvert or open independently of it to the outside
(Figs. 1.22 , 2.7b (D, E), 2.46 , and 2.47 ) (summarized in
Ostrovsky et al. 2006 , 2009b ; Ostrovsky 2008a , b ). Brood
sacs belong to the third group of brood chambers as defi ned
above. Beania bilaminata (Fig. 1.22 ) and species of
Cauloramphus are intermediate with respect to ovicells and
internal brood sacs (Figs. 2.6b (E), 2.7b (C), and 2.25B )
(Ostrovsky et al. 2007 , 2009a ). All three elements of the ovicell are present – kenozooidal ooecium (reduced, cap-like),
incubation cavity and ooecial vesicle – and their brooding
apparatus is fairly similar to the immersed ovicells of the
calloporid Crassimarginatella sp. (cf. Figs. 2.7b (B, C) and
2.25A, B ). It should be emphasized once again that the
much-used phrase “vestigial/reduced ovicell” (see, for
instance, Harmer 1926 ; Hastings 1945 ) is inaccurate, since
the brood cavity, as part of the ovicell, is always well developed. The term “vestigial” [small or rudimentary] can be true
only of the ooecium. The brood chambers of Cauloramphus
and Beania bilaminata , although evolved from ovicells, are
internal brood sacs that have retained vestigial ooecia (see
also Ostrovsky et al. 2007 ).
In some taxa (e.g. Adeonidae) internal brooding is
combined with changes in cystid shape and size, being
an example of sexual zooidal dimorphism. For such zooids
(often enlarged) it would be correct to use the term “autozooidal polymorph with an internal brood sac” (see Sect. 2.3.3 ).
“Endotoichal” ovicells (Levinsen 1902 , 1909 ), known
only in the family Cellariidae, are a special case (see
Sect. 2.3.2 ). Their anatomy was fi rst described by Calvet
( 1900 ) and recently restudied (Ostrovsky 2009 ). The skeletal
walls of the brood chamber belong to 1–3 distal zooids,
whereas the embryo is enveloped by the modifi ed ooecial
vesicle formed by the maternal zooid (Figs. 1.19B–D , 1.20A,
B, D , 2.7b (F), 2.38 , and 2.39 ). Basically, the endotoichal ovicell is a highly modifi ed endozooidal ovicell.
2.2.3 Ovicell Closure
Yet another approach to ovicell classifi cation is based on
their closure method. Ovicells that are closed only by the
ooecial vesicle are called “acleithral” (see Fig. 1 in Introduction,
Figs. 1.17 , 1.18A , 2.4 , 2.6a (A, B, E, F), b (C), 2.7a (A, B, G),
b (A), 2.8A, F , 2.14B , 2.15A, B , 2.16 , 2.22A , 2.23 , 2.30A ,
2.31B , 2.32 , 2.36 , 2.44A , and 2.63B, C ), whereas those
closed by the zooidal operculum (plus the underlying ooecial
vesicle or non-calcifi ed distal wall of the maternal zooid) are
called “cleithral” (Figs. 1.24A , 1.25 , 1.27D , 1.28A, B , 1.30 ,
1.32A, B , 1.36 , 2.6a (C, D), b (A, B, D), 2.7a (C, F, H), b (B),
2.8B, D , 2.22B , 2.24 , 2.25A , 2.29 , 2.30B , 2.33A, F , 2.34 ,
2.41 , 2.44B , 2.45 , 2.49B , 2.57C , 2.60B, C, E , 2.61 , and
2.63A, D ). An intermediate position pertains to “semicleithral” ovicells (Figs. 1.28C, D , 2.7a (I), 2.8C , and 2.28 ),
in which the zooidal operculum closes the ovicell opening
incompletely (Ostrovsky 2008b ). Isolation of the brood cavity from the external medium is here provided by the ooecial
vesicle since the distal edge of the operculum does not reach
the margin of the ovicell opening. I have encountered a number of examples of such closure in fi xed material, and both
cleithral and semicleithral ovicells were sometimes found in
the same species. Thus, one should be alert to the possibility
of confusion caused by shrinkage of the frontal membrane or
ascus wall during fi xation or drying, because the operculum
is connected to this membrane/wall (also discussed in Cook
1977a ). It should be stressed that the more the brood cavity
is immersed, the greater is the probability of it being semicleithral or cleithral.
In contrast to species with cleithral ovicells that raise their
opercula during larval extrusion (Figs. 2.7a (C) and 2.8B )
(e.g. Smittipora levinseni , see Cook 1985 ), in Pacifi cincola
insculpta (as ‘ Hippodiplosia ’) and Fenestrulina miramara
(as F. malusii ) the operculum is lowered during larval release
(see Nielsen 1981 ) (Figs. 2.8D and 2.45 ). This variant of the
cleithral type can be termed “subcleithral,” as modifi ed by
Ryland ( 1968 , p. 233) [who described this type as having
two “closed positions, the upper sealing off the ovicell, the
lower sealing the [zooidal] orifi ce only”] and based on the
term “subcleithrian(s)” of Canu and Bassler ( 1920 ). Ryland
( 1968 ) stated that this type exists in Pentapora (see also
Carson 1978 ). Observations on living material are necessary
to distinguish this type of ovicell closure.
The term “pseudocleithral”, proposed by Ryland ( 1968 ),
describes a situation in which the operculum closes the ovicell opening for a brief moment during polypide protrusion
or retraction. While the tentacle crown is everted, the operculum maintains a vertical position. When the tentacle crown is
retracted, the operculum closes the zooidal orifi ce and the
ovicell opening is plugged by the ooecial vesicle (Fig. 2.8F ).
Judging from the length and position of the operculum, this
variant of the acleithral type possibly exists in Schizomavella
cuspidata. In two other species of this genus ( S. lineata, S.
mamillata ) I found acleithral ovicells which, judging from
the position of the operculum, cannot be closed by it during
excursions of the polypide.
Levinsen ( 1909 ) was the fi rst to note that, in some ovicells,
the opening is not closed at all, since the zooidal operculum is
distant and an ooecial plug is absent (Figs. 2.6b (F), 2.8E , and 2.42 )
2 Cheilostome Brood Chambers: Structure, Formation, Evolution
“Peristomial” ovicells (Levinsen 1902 , 1909 ), in which the
ooecial capsule is incorporated into the zooidal peristome,
comprise subimmersed or sometimes endozooidal types
(Figs. 2.7a (D, E) and 2.37 ).
Further immersion of the brood cavity in the maternal
zooid, concurrent with reduction of the ooecial fold, eventually results in “internal sacs,” “internal embryo sacs,” or
“membranous diverticula” (Waters 1912 ; Ström 1977 ; Cook
1979 ), in which the cavity can be connected with that of
the introvert or open independently of it to the outside
(Figs. 1.22 , 2.7b (D, E), 2.46 , and 2.47 ) (summarized in
Ostrovsky et al. 2006 , 2009b ; Ostrovsky 2008a , b ). Brood
sacs belong to the third group of brood chambers as defi ned
above. Beania bilaminata (Fig. 1.22 ) and species of
Cauloramphus are intermediate with respect to ovicells and
internal brood sacs (Figs. 2.6b (E), 2.7b (C), and 2.25B )
(Ostrovsky et al. 2007 , 2009a ). All three elements of the ovicell are present – kenozooidal ooecium (reduced, cap-like),
incubation cavity and ooecial vesicle – and their brooding
apparatus is fairly similar to the immersed ovicells of the
calloporid Crassimarginatella sp. (cf. Figs. 2.7b (B, C) and
2.25A, B ). It should be emphasized once again that the
much-used phrase “vestigial/reduced ovicell” (see, for
instance, Harmer 1926 ; Hastings 1945 ) is inaccurate, since
the brood cavity, as part of the ovicell, is always well developed. The term “vestigial” [small or rudimentary] can be true
only of the ooecium. The brood chambers of Cauloramphus
and Beania bilaminata , although evolved from ovicells, are
internal brood sacs that have retained vestigial ooecia (see
also Ostrovsky et al. 2007 ).
In some taxa (e.g. Adeonidae) internal brooding is
combined with changes in cystid shape and size, being
an example of sexual zooidal dimorphism. For such zooids
(often enlarged) it would be correct to use the term “autozooidal polymorph with an internal brood sac” (see Sect. 2.3.3 ).
“Endotoichal” ovicells (Levinsen 1902 , 1909 ), known
only in the family Cellariidae, are a special case (see
Sect. 2.3.2 ). Their anatomy was fi rst described by Calvet
( 1900 ) and recently restudied (Ostrovsky 2009 ). The skeletal
walls of the brood chamber belong to 1–3 distal zooids,
whereas the embryo is enveloped by the modifi ed ooecial
vesicle formed by the maternal zooid (Figs. 1.19B–D , 1.20A,
B, D , 2.7b (F), 2.38 , and 2.39 ). Basically, the endotoichal ovicell is a highly modifi ed endozooidal ovicell.
2.2.3 Ovicell Closure
Yet another approach to ovicell classifi cation is based on
their closure method. Ovicells that are closed only by the
ooecial vesicle are called “acleithral” (see Fig. 1 in Introduction,
Figs. 1.17 , 1.18A , 2.4 , 2.6a (A, B, E, F), b (C), 2.7a (A, B, G),
b (A), 2.8A, F , 2.14B , 2.15A, B , 2.16 , 2.22A , 2.23 , 2.30A ,
2.31B , 2.32 , 2.36 , 2.44A , and 2.63B, C ), whereas those
closed by the zooidal operculum (plus the underlying ooecial
vesicle or non-calcifi ed distal wall of the maternal zooid) are
called “cleithral” (Figs. 1.24A , 1.25 , 1.27D , 1.28A, B , 1.30 ,
1.32A, B , 1.36 , 2.6a (C, D), b (A, B, D), 2.7a (C, F, H), b (B),
2.8B, D , 2.22B , 2.24 , 2.25A , 2.29 , 2.30B , 2.33A, F , 2.34 ,
2.41 , 2.44B , 2.45 , 2.49B , 2.57C , 2.60B, C, E , 2.61 , and
2.63A, D ). An intermediate position pertains to “semicleithral” ovicells (Figs. 1.28C, D , 2.7a (I), 2.8C , and 2.28 ),
in which the zooidal operculum closes the ovicell opening
incompletely (Ostrovsky 2008b ). Isolation of the brood cavity from the external medium is here provided by the ooecial
vesicle since the distal edge of the operculum does not reach
the margin of the ovicell opening. I have encountered a number of examples of such closure in fi xed material, and both
cleithral and semicleithral ovicells were sometimes found in
the same species. Thus, one should be alert to the possibility
of confusion caused by shrinkage of the frontal membrane or
ascus wall during fi xation or drying, because the operculum
is connected to this membrane/wall (also discussed in Cook
1977a ). It should be stressed that the more the brood cavity
is immersed, the greater is the probability of it being semicleithral or cleithral.
In contrast to species with cleithral ovicells that raise their
opercula during larval extrusion (Figs. 2.7a (C) and 2.8B )
(e.g. Smittipora levinseni , see Cook 1985 ), in Pacifi cincola
insculpta (as ‘ Hippodiplosia ’) and Fenestrulina miramara
(as F. malusii ) the operculum is lowered during larval release
(see Nielsen 1981 ) (Figs. 2.8D and 2.45 ). This variant of the
cleithral type can be termed “subcleithral,” as modifi ed by
Ryland ( 1968 , p. 233) [who described this type as having
two “closed positions, the upper sealing off the ovicell, the
lower sealing the [zooidal] orifi ce only”] and based on the
term “subcleithrian(s)” of Canu and Bassler ( 1920 ). Ryland
( 1968 ) stated that this type exists in Pentapora (see also
Carson 1978 ). Observations on living material are necessary
to distinguish this type of ovicell closure.
The term “pseudocleithral”, proposed by Ryland ( 1968 ),
describes a situation in which the operculum closes the ovicell opening for a brief moment during polypide protrusion
or retraction. While the tentacle crown is everted, the operculum maintains a vertical position. When the tentacle crown is
retracted, the operculum closes the zooidal orifi ce and the
ovicell opening is plugged by the ooecial vesicle (Fig. 2.8F ).
Judging from the length and position of the operculum, this
variant of the acleithral type possibly exists in Schizomavella
cuspidata. In two other species of this genus ( S. lineata, S.
mamillata ) I found acleithral ovicells which, judging from
the position of the operculum, cannot be closed by it during
excursions of the polypide.
Levinsen ( 1909 ) was the fi rst to note that, in some ovicells,
the opening is not closed at all, since the zooidal operculum is
distant and an ooecial plug is absent (Figs. 2.6b (F), 2.8E , and 2.42 )
2 Cheilostome Brood Chambers: Structure, Formation, Evolution