135
the vesicle. Muscular contractions of the ooecial vesicle possibly also occur during oviposition.
The ovicell of Corbulella maderensis is cleithral, its
opening closed by the zooidal operculum and the underlying
ooecial vesicle – a small outgrowth of the upper part of the
distal wall of the maternal autozooid (Figs. 2.7a (C), 2.8B ,
and 2.22B ). The lower part of the vesicle may protrude
slightly into the brood cavity. It lacks a sclerite and is fi lled
with numerous funicular cells that give it a parenchymatose
appearance in some sections. Two thin muscle bundles attach
to the distal wall in upper and middle parts of the vesicle
(Ostrovsky et al. 2009a ).
2.3.1.4 Development of Hyperstomial
Ovicells in Recent Calloporids
The fertile maternal autozooid initially forms a distal bud,
which later results in the distal zooid with the ooecium
(Fig. 2.18A–D, F ). Sometimes an ovicell is developed and
even starts brooding long before the formation of the daughter zooid is completed.
In general, the ooecium originates as a vertical outgrowth
of the membranous frontal wall in the proximal part of the
developing distal zooid (type 1). The ooecial fold is produced by intussusception in the same manner as an autozooid (reviewed in Ryland 1976 ), recognizable as an expansion
of cuticle by a group or zone of dividing epithelial cells. The
fi rst indication of ovicellogenesis is a localized calcifi cation
of the frontal wall of the distal zooid. Starting from the
upper edge of the transverse wall dividing maternal and distal zooids, it spreads centrifugally, giving the impression, in
the early stages, of two rounded plates (often referred to as
the “ooecial rudiment”) (Figs. 2.13B and 2.18A ). The plates
originate independently and may differ in size. Eventually
they merge to form a bilobate plate often with a weakly
expressed medial suture or low keel. This calcifi ed zone
enlarges further to form a concave area, the ovicell fl oor
(Fig. 2.18B–D ), i.e. the proximal part of the entooecium. At
this stage the bilobate shape of calcifi cation is normally lost,
although the trace left by the two merged plates often can be
seen (Ostrovsky and Schäfer 2003 ; Ostrovsky et al. 2003 ,
2009a ).
Contemporaneous with formation of the frontally visible
proximal part of entooecial calcifi cation is an additional
calcifi ed layer underlying it, with a different crystalline
structure (Fig. 2.20A–C ). This layer starts from the transverse and lateral walls of the distal zooid bud and, together,
the two layers form the more-or-less fl at-shelved ovicell
fl oor (Fig. 2.18B ). This underlying layer was fi rst described
by Nielsen ( 1985 ) in Tegella aquilirostris , Scrupocellaria
varians and Tricellaria occidentalis and referred to as a
cryptocyst because of its shape and position. This layer
spreads downwards to cover the vertical walls of the zooid,
and its external borders are usually clearly discernible
(Fig. 2.20 ) (Ostrovsky et al. 2003 ).
The fully formed concave ovicell fl oor thus consists of a
very thin cuticle and two calcifi ed layers, its frontally
expressed exterior surface nominally a gymnocyst. At its
periphery the ovicell fl oor is bordered by a protruding
membranous fold of future ooecium (Figs. 2.18D–F and
2.20A–C ), the coelomic lumen of which communicates with
the visceral coelom of the distal zooid via an arched communication slit that later closes (Fig. 2.21 ). The ooecial fold
grows upwards, its calcifi cation being slightly retarded
(Figs. 2.18D–F , 2.19A, B , and 2.20A–C ). Calcifi cation of the
ectooecium starts from the lateral walls of the distal zooid
that are continuous with the base of the ooecial fold. As the
ooecium grows, calcifi cation of the vertical ectooecial
wall (also of two calcifi ed layers) takes the form of two symmetrical elongated lateral lobes that merge to form a distal
hood over the developing entooecium. A thin coelomic
lumen is retained between the entooecium and the ectooecium (Fig. 2.20C, F ) (Ostrovsky et al. 2003 ).
In the process of forming the ovicell roof, the upper part
of the ooecial fold generally develops evenly, with centripetal calcifi cation (Figs. 2.13B and 2.19A ). There can be
exceptions, encountered, for example, in Callopora lineata
and Tegella armifera in which the ooecial roof was formed by
fusion of two fl at lateral lobes emerging late in development
(Fig. 2.19B–D ). Normally these lobes, initially non- calcifi ed,
grow towards each other and fuse leaving no trace of a
median suture (Ostrovsky et al. 2003 ). It is possible that the
above-mentioned medial groove found on the inner entooecial
surface of a specimen of C . lineata formed in this way.
Calcifi cation of ooecial walls proceeds in tandem with
development of the fold with only a slight delay, following
its growth except for non-calcifi ed areas of the ectooecium (Fig. 2.19 ). In most of the species of Callopora and
Tegella examined in the course of this study, as well as in
Amphiblestrum inermis , the ooecium is associated with an
adventitious avicularian chamber (Figs. 2.13A , 2.14A, C, F ,
2.15A , 2.16 , 2.19 , and 2.22A ). In these cases, its interior wall
(cryptocyst) forms the vertical ectooecial wall, separating the
coeloms of the ooecium and the avicularium (Fig. 2.20D–F )
(Nielsen 1985 ; Ostrovsky et al. 2003 , 2009a ).
The ooecial vesicle is formed at the same time as the
ooecial fold, as an outgrowth of the upper part of the distal
wall of the maternal autozooid (Fig. 2.20C, F ) (Silén 1945 ;
Ostrovsky and Schäfer 2003 ).
2.3.1.5 Subimmersed Ovicells
Formed at the expense of the distal autozooid (type 1, category
A), ovicells of Valdemunitella lata are traditionally described
as prominent and bilobate (cf. Gordon 1986 ). Since more than
half the volume of the brood cavity is below the colony surface
2.3 Structure and Development of Brood Chambers in Cheilostomata
the vesicle. Muscular contractions of the ooecial vesicle possibly also occur during oviposition.
The ovicell of Corbulella maderensis is cleithral, its
opening closed by the zooidal operculum and the underlying
ooecial vesicle – a small outgrowth of the upper part of the
distal wall of the maternal autozooid (Figs. 2.7a (C), 2.8B ,
and 2.22B ). The lower part of the vesicle may protrude
slightly into the brood cavity. It lacks a sclerite and is fi lled
with numerous funicular cells that give it a parenchymatose
appearance in some sections. Two thin muscle bundles attach
to the distal wall in upper and middle parts of the vesicle
(Ostrovsky et al. 2009a ).
2.3.1.4 Development of Hyperstomial
Ovicells in Recent Calloporids
The fertile maternal autozooid initially forms a distal bud,
which later results in the distal zooid with the ooecium
(Fig. 2.18A–D, F ). Sometimes an ovicell is developed and
even starts brooding long before the formation of the daughter zooid is completed.
In general, the ooecium originates as a vertical outgrowth
of the membranous frontal wall in the proximal part of the
developing distal zooid (type 1). The ooecial fold is produced by intussusception in the same manner as an autozooid (reviewed in Ryland 1976 ), recognizable as an expansion
of cuticle by a group or zone of dividing epithelial cells. The
fi rst indication of ovicellogenesis is a localized calcifi cation
of the frontal wall of the distal zooid. Starting from the
upper edge of the transverse wall dividing maternal and distal zooids, it spreads centrifugally, giving the impression, in
the early stages, of two rounded plates (often referred to as
the “ooecial rudiment”) (Figs. 2.13B and 2.18A ). The plates
originate independently and may differ in size. Eventually
they merge to form a bilobate plate often with a weakly
expressed medial suture or low keel. This calcifi ed zone
enlarges further to form a concave area, the ovicell fl oor
(Fig. 2.18B–D ), i.e. the proximal part of the entooecium. At
this stage the bilobate shape of calcifi cation is normally lost,
although the trace left by the two merged plates often can be
seen (Ostrovsky and Schäfer 2003 ; Ostrovsky et al. 2003 ,
2009a ).
Contemporaneous with formation of the frontally visible
proximal part of entooecial calcifi cation is an additional
calcifi ed layer underlying it, with a different crystalline
structure (Fig. 2.20A–C ). This layer starts from the transverse and lateral walls of the distal zooid bud and, together,
the two layers form the more-or-less fl at-shelved ovicell
fl oor (Fig. 2.18B ). This underlying layer was fi rst described
by Nielsen ( 1985 ) in Tegella aquilirostris , Scrupocellaria
varians and Tricellaria occidentalis and referred to as a
cryptocyst because of its shape and position. This layer
spreads downwards to cover the vertical walls of the zooid,
and its external borders are usually clearly discernible
(Fig. 2.20 ) (Ostrovsky et al. 2003 ).
The fully formed concave ovicell fl oor thus consists of a
very thin cuticle and two calcifi ed layers, its frontally
expressed exterior surface nominally a gymnocyst. At its
periphery the ovicell fl oor is bordered by a protruding
membranous fold of future ooecium (Figs. 2.18D–F and
2.20A–C ), the coelomic lumen of which communicates with
the visceral coelom of the distal zooid via an arched communication slit that later closes (Fig. 2.21 ). The ooecial fold
grows upwards, its calcifi cation being slightly retarded
(Figs. 2.18D–F , 2.19A, B , and 2.20A–C ). Calcifi cation of the
ectooecium starts from the lateral walls of the distal zooid
that are continuous with the base of the ooecial fold. As the
ooecium grows, calcifi cation of the vertical ectooecial
wall (also of two calcifi ed layers) takes the form of two symmetrical elongated lateral lobes that merge to form a distal
hood over the developing entooecium. A thin coelomic
lumen is retained between the entooecium and the ectooecium (Fig. 2.20C, F ) (Ostrovsky et al. 2003 ).
In the process of forming the ovicell roof, the upper part
of the ooecial fold generally develops evenly, with centripetal calcifi cation (Figs. 2.13B and 2.19A ). There can be
exceptions, encountered, for example, in Callopora lineata
and Tegella armifera in which the ooecial roof was formed by
fusion of two fl at lateral lobes emerging late in development
(Fig. 2.19B–D ). Normally these lobes, initially non- calcifi ed,
grow towards each other and fuse leaving no trace of a
median suture (Ostrovsky et al. 2003 ). It is possible that the
above-mentioned medial groove found on the inner entooecial
surface of a specimen of C . lineata formed in this way.
Calcifi cation of ooecial walls proceeds in tandem with
development of the fold with only a slight delay, following
its growth except for non-calcifi ed areas of the ectooecium (Fig. 2.19 ). In most of the species of Callopora and
Tegella examined in the course of this study, as well as in
Amphiblestrum inermis , the ooecium is associated with an
adventitious avicularian chamber (Figs. 2.13A , 2.14A, C, F ,
2.15A , 2.16 , 2.19 , and 2.22A ). In these cases, its interior wall
(cryptocyst) forms the vertical ectooecial wall, separating the
coeloms of the ooecium and the avicularium (Fig. 2.20D–F )
(Nielsen 1985 ; Ostrovsky et al. 2003 , 2009a ).
The ooecial vesicle is formed at the same time as the
ooecial fold, as an outgrowth of the upper part of the distal
wall of the maternal autozooid (Fig. 2.20C, F ) (Silén 1945 ;
Ostrovsky and Schäfer 2003 ).
2.3.1.5 Subimmersed Ovicells
Formed at the expense of the distal autozooid (type 1, category
A), ovicells of Valdemunitella lata are traditionally described
as prominent and bilobate (cf. Gordon 1986 ). Since more than
half the volume of the brood cavity is below the colony surface
2.3 Structure and Development of Brood Chambers in Cheilostomata