164
they are rather common in calloporids, the most ancient family of brooding cheilostomes. The lack of the ooecial vesicle,
sclerite and/or musculature in some cheilostomes would
seem to be secondary. As noted above, the disappearance of
the ooecial vesicle may have been associated with the transition to cleithral ovicells.
The loss of the sclerite (substituted in some species by a
thickened cuticle) in many bryozoans was not associated
with loss of the musculature. Flustrids with a weakly developed ooecial vesicle have no sclerite ( Isosecurifl ustra
angusta , Klugefl ustra antarctica ). Apparently, the effort necessary for vesicle retraction is not great and thus there is no
need for a thickened structure for muscle attachment. Species
with and without the sclerite occur in most families, which
may indicate yet another evolutionary trend. However, the
information presently available is insuffi cient for any farreaching conclusions.
2.4.8.7 Evolution of Peristomial Ovicells
Peristomial ovicells are known in the Margarettidae
( Margaretta ), Lacernidae ( Cylindroporella ), Lekythoporidae
( Poecilopora ) and Cribrilinidaе ( Haplocephalopora ,
Pachydera ) (Lang 1916 ; Voigt 1993 ; Ostrovsky, unpublished
data) (Figs. 2.7a (D, Е) and 2.37 ). The patchy distribution of
these taxa in the phylogenetic tree indicates that the transition from hyperstomial to peristomial ovicells is a distinct
evolutionary trend originating independently in at least four
distant families. Evolution of the peristomial ovicell was
associated with the fusion of the ooecium and the peristome
(the collar- or tube-like calcifi ed wall around the orifi ce of
the maternal zooid). Peristomes evolved as modifi cations of
the zooidal orifi ce and/or frontal shield, and were probably
protective structures preventing predation through forcing of
the operculum.
Since the ooecium is situated near the zooidal orifi ce, it
is naturally incorporated into the peristome wall, and the
brood chamber cannot open directly to the environment
but into the peristome cavity. It may be noted that formation of peristomial ovicells may be accompanied by
immersion of the brood cavity into the colony ( Margaretta ,
Poecilopora ).
2.4.8.8 Proximal Displacement and Reduction
of the Ooecial Base
Levinsen ( 1909 , рp. 62–63) was the fi rst to note the difference in the size of the “common wall for zooecium [zooid]
and ooecium”, i.e. the size of the ooecial base in different,
sometimes congeneric, cheilostomes. He observed that the
“common wall” is large in some species whereas in others
the ooecium has a “narrow … pedunculate basal part,” terming the former ooecia “dependent” and the latter “independent” (see Sect. 2.2 ).
Since the ooecia of the most ancient cheilostome
brooders (including Calloporidae) are developed as an
arch-like fold on the frontal gymnocyst of the distal zooid,
their base is represented by the ovicell fl oor surrounded
by the basal part of the ooecial vertical walls. Thus, the
ovicell fl oor (horizontal part of the entooecium) constitutes a considerable part of the frontal wall of the ooecium-producing zooid. In many species, however, this
“common wall” is much smaller or absent and the ooecium and the frontal wall of the distal zooid are connected
via a narrow (and often very short) “stalk” with calcifi ed
walls surrounding a communication pore. The pattern of
distribution of these two structural variants across
Cheilostomata points to a trend, in some cases presumably associated with the evolution of new types of frontal
shield and the reduction of the proximal area involved in
the formation of the ooecium (see Sect. 2.4.7 ). In the
course of this transformation, the broad ooecial base,
shaped as an arched fold (Fig. 2.18D–F ) became a “double disc” (a fold with a narrow base) (Fig. 2.40C–F ).
This trend, evident within superfamilies, families and
genera, appears repeatedly within the Cheilostomata. For
instance, the proximal position of the narrow ooecial base is
characteristic of the Hiantoporidae and some Bugulidae
among anascans and of a number of umbonulomorph
(Fig. 2.41 ) and lepraliomorph ascophorans with lepralielliform ooecia (see Sects. 2.3.2 and 2.4.7 ). In all of them the
developing ooecium has the shape of a “double disc” – the
displacement of the ooecial base towards the transverse wall
between maternal and distal zooids precludes the development of a “broad” ooecial fold such as is observed in
Callopora , for example. Further, ovicell fl oor formation
starts with single (unpaired) rudiment of calcifi cation, and
the general reduction of the ooecial base may have been also
a reason for the secondary acquisition of the shape of the
initial calcifi cation.
Together with the reduction of the ooecial base a communication slit transforms to a central pore. Whereas most
Bugulidae have such a pore, Nordgaardia cornucopioides
has a communication slit and a broader base, indicating the
plesiomorphic state of this character. Similarly, both the
communication slit and pore occur in different species of
Smittina (see Sect. 2.4.7 ).
Reduction of the ooecial base and the transformation of a
slit to a pore in ascophorans with lepralielliform ooecia was
also accompanied by a proximal displacement of the ooecial
communication pore, enlargement of the horizontal ectooecial part (and thus its contact area with the frontal shield of
the distal zooid) (Fig. 2.41 ) and, in some cases, the character
of ovicellogenesis. Such correlations among these three
characters exist in the Bryocryptellidae, Smittinidae and
Bitectiporidae. It is only in rare cases (e.g. in Hippoporina
2 Cheilostome Brood Chambers: Structure, Formation, Evolution
they are rather common in calloporids, the most ancient family of brooding cheilostomes. The lack of the ooecial vesicle,
sclerite and/or musculature in some cheilostomes would
seem to be secondary. As noted above, the disappearance of
the ooecial vesicle may have been associated with the transition to cleithral ovicells.
The loss of the sclerite (substituted in some species by a
thickened cuticle) in many bryozoans was not associated
with loss of the musculature. Flustrids with a weakly developed ooecial vesicle have no sclerite ( Isosecurifl ustra
angusta , Klugefl ustra antarctica ). Apparently, the effort necessary for vesicle retraction is not great and thus there is no
need for a thickened structure for muscle attachment. Species
with and without the sclerite occur in most families, which
may indicate yet another evolutionary trend. However, the
information presently available is insuffi cient for any farreaching conclusions.
2.4.8.7 Evolution of Peristomial Ovicells
Peristomial ovicells are known in the Margarettidae
( Margaretta ), Lacernidae ( Cylindroporella ), Lekythoporidae
( Poecilopora ) and Cribrilinidaе ( Haplocephalopora ,
Pachydera ) (Lang 1916 ; Voigt 1993 ; Ostrovsky, unpublished
data) (Figs. 2.7a (D, Е) and 2.37 ). The patchy distribution of
these taxa in the phylogenetic tree indicates that the transition from hyperstomial to peristomial ovicells is a distinct
evolutionary trend originating independently in at least four
distant families. Evolution of the peristomial ovicell was
associated with the fusion of the ooecium and the peristome
(the collar- or tube-like calcifi ed wall around the orifi ce of
the maternal zooid). Peristomes evolved as modifi cations of
the zooidal orifi ce and/or frontal shield, and were probably
protective structures preventing predation through forcing of
the operculum.
Since the ooecium is situated near the zooidal orifi ce, it
is naturally incorporated into the peristome wall, and the
brood chamber cannot open directly to the environment
but into the peristome cavity. It may be noted that formation of peristomial ovicells may be accompanied by
immersion of the brood cavity into the colony ( Margaretta ,
Poecilopora ).
2.4.8.8 Proximal Displacement and Reduction
of the Ooecial Base
Levinsen ( 1909 , рp. 62–63) was the fi rst to note the difference in the size of the “common wall for zooecium [zooid]
and ooecium”, i.e. the size of the ooecial base in different,
sometimes congeneric, cheilostomes. He observed that the
“common wall” is large in some species whereas in others
the ooecium has a “narrow … pedunculate basal part,” terming the former ooecia “dependent” and the latter “independent” (see Sect. 2.2 ).
Since the ooecia of the most ancient cheilostome
brooders (including Calloporidae) are developed as an
arch-like fold on the frontal gymnocyst of the distal zooid,
their base is represented by the ovicell fl oor surrounded
by the basal part of the ooecial vertical walls. Thus, the
ovicell fl oor (horizontal part of the entooecium) constitutes a considerable part of the frontal wall of the ooecium-producing zooid. In many species, however, this
“common wall” is much smaller or absent and the ooecium and the frontal wall of the distal zooid are connected
via a narrow (and often very short) “stalk” with calcifi ed
walls surrounding a communication pore. The pattern of
distribution of these two structural variants across
Cheilostomata points to a trend, in some cases presumably associated with the evolution of new types of frontal
shield and the reduction of the proximal area involved in
the formation of the ooecium (see Sect. 2.4.7 ). In the
course of this transformation, the broad ooecial base,
shaped as an arched fold (Fig. 2.18D–F ) became a “double disc” (a fold with a narrow base) (Fig. 2.40C–F ).
This trend, evident within superfamilies, families and
genera, appears repeatedly within the Cheilostomata. For
instance, the proximal position of the narrow ooecial base is
characteristic of the Hiantoporidae and some Bugulidae
among anascans and of a number of umbonulomorph
(Fig. 2.41 ) and lepraliomorph ascophorans with lepralielliform ooecia (see Sects. 2.3.2 and 2.4.7 ). In all of them the
developing ooecium has the shape of a “double disc” – the
displacement of the ooecial base towards the transverse wall
between maternal and distal zooids precludes the development of a “broad” ooecial fold such as is observed in
Callopora , for example. Further, ovicell fl oor formation
starts with single (unpaired) rudiment of calcifi cation, and
the general reduction of the ooecial base may have been also
a reason for the secondary acquisition of the shape of the
initial calcifi cation.
Together with the reduction of the ooecial base a communication slit transforms to a central pore. Whereas most
Bugulidae have such a pore, Nordgaardia cornucopioides
has a communication slit and a broader base, indicating the
plesiomorphic state of this character. Similarly, both the
communication slit and pore occur in different species of
Smittina (see Sect. 2.4.7 ).
Reduction of the ooecial base and the transformation of a
slit to a pore in ascophorans with lepralielliform ooecia was
also accompanied by a proximal displacement of the ooecial
communication pore, enlargement of the horizontal ectooecial part (and thus its contact area with the frontal shield of
the distal zooid) (Fig. 2.41 ) and, in some cases, the character
of ovicellogenesis. Such correlations among these three
characters exist in the Bryocryptellidae, Smittinidae and
Bitectiporidae. It is only in rare cases (e.g. in Hippoporina
2 Cheilostome Brood Chambers: Structure, Formation, Evolution