147
with the operculum above it. The distal margin of the operculum is situated close to the proximal border of the ovicell
but does not adjoin it.
The ovicells of Thalamoporella are distinctive. Levinsen
( 1902 , p. 15) referred to them as “epistomial” but later
considered them to be hyperstomial (Levinsen 1909 ).
Harmer ( 1926 , p. 291) suggested that they are non-homologous to hyperstomial ovicells in other Cheilostomata, proposing that they evolved from the “adoral tubercles” of the
maternal zooid. Marcus ( 1941a , pl. 4, fi g. 11) presented the
stages of ovicellogenesis and a schematic of a longitudinal
section of the ovicelled autozooid of T . evelinae . It gives the
impression that the ooecium consists of three walls in this
species. Marcus did not show any communication organs
between ooecial and zooidal coeloms.
My study of Thalamoporella sp. showed that the ooecium of the cleithral ovicell is formed from the maternal
autozooid, which has a larger orifi ce than non-ovicelled
zooids (Fig. 2.49A, D ). This is a special type of bivalved
ovicell, formed at the frontal surface of the maternal autozooid around its orifi ce (see also Levinsen 1902 ; Harmer
1926 ). The intermediate stage of ovicellogenesis superfi -
cially resembles ooecial-fold development in calloporids
(Fig. 2.49D ). The calcifi ed ooecium results from the fusion
of two symmetrical, hollow hemispherical lobes along the
midline of the ooecium, leaving a medial suture visible
externally and inter nally (Fig. 2.49B, C, E, F , see also pl. 4,
fi g. 7 in Marcus 1941a ). In contrast with the calloporid
Bryocalyx cinnameus , in which the ooecial lobes are separated by a double longitudinal septum in the distal part of
the ooecium, the lobes in Thalamoporella are separated by a
septum only at the ooecial base (Fig. 2.49E ); in the upper
part there is no septum and the ooecial roof is thus complete
even though the medial suture is retained (Fig. 2.49F ).
Waters ( 1909 , p. 142) termed the ovicells of Thalamoporella
“bilobate,” but also stated that “there is no complete divisional wall in” them. The nature of the internal “wall” in the
ooecium seen in the above- mentioned illustration of Marcus
is puzzling since it was not shown in another of Marcus’s
fi gures ( 1941a , pl. 5, fi g. 12a).
The ooecial coelom communicates directly with the
visceral coelom of the maternal autozooid via two large,
symmetrical arched openings at the sides of its aperture
(Fig. 2.49C ). Thus, in this case, although the ooecium is
formed at the expense of the maternal autozooid, it is not a
kenozooid but a paired outgrowth of the frontal zooidal wall.
2.3.5 Acanthostegal Brood Chambers
These structures, made of fl attened mural spines, are known
only in three living species of Tendridae (Cheilostomata)
(Hincks 1892 ; Levinsen 1909 ; Ostrovsky and Taylor
2005a ). Repiachoff ( 1875 ), Reinhard ( 1875 ) and Ostroumoff
( 1886a , b , c ) studied them in Tendra zostericola . Although
mistaken in their understanding of the construction of these
brood chambers, and believing that embryos were developed inside the body cavity of specialized zooids, Repiahoff
( 1875 ) nevertheless suggested that they play the role of ovicells, and Reinhard ( 1875 , p. 25) stated that “ Tendra will
represent a transition between bryozoans without ovicells
to those that possess them”. Ostroumoff ( 1886a ) was the
fi rst to understand that the embryos are brooded in the
space [epistege] between the frontal membrane and the
over-arching spines in this species (see also Appendix I for
historical review).
In Tendra zostericola , the brooding zooid produces a pair
of articulated oral spines and, at the mural edge, two (sometimes one) lateral rows of horizontally inclined inarticulate
spines that are fl attened at the base. These long, pointed
spines closely adjoin each other and the spines of the opposite row, forming the acanthostegal (literally “spine-roofed”)
brood chamber; the space between it and the underlying
frontal membrane is the brood cavity (Figs. 2.50 and 2.59A ).
Each lateral row typically consists of 10–15 spines (up to 17
(Repiachoff 1875 ; Levinsen 1909 ), 13–18 (Occipinti Ambrogi
1981 ; Occhipinti Ambrogi and d’Hondt 1981 )). The proximal edge is free of spines, providing an opening for oviposition and larval release (Fig. 2.50B ). It may remain open but
is usually closed by the operculum of the proximal (maternal)
autozooid, as in the case of the cleithral ovicells of other
cheilostomes.
The so-called brooding “zooids” of Heteroecium amplectens are a complex of two zooids – the proximal (maternal)
autozooid (apparently an autozooidal polymorph) and a distal kenozooid (Figs. 2.51A, B and 2.59B ). At the mural edge
of the latter, up to 15–17 fl attened inarticulate spines form
the roof of the brood chamber, similar to the situation in
Tendra . They closely adjoin each other, leaving no spaces
between, their ends fusing along the midline of the kenozooid to form a low longitudinal keel. The brood chamber has
the shape of an elongated hemisphere with a single proximal
opening closed by the operculum of the maternal zooid, similar to cleithral closure in other cheilostomes. The broodcavity fl oor is calcifi ed, except for a proximal membranous
area where there are two lateral outgrowths (Fig. 2.51C, D
and 2.59B ) facing the kenozooidal coelomic cavity. It may
be conjectured that this area of kenozooidal frontal wall
is a rudiment of the frontal membrane of the autozooid,
with parietal musculature. The lateral outgrowths would
then serve for attachment of these muscles (Ostrovsky and
Taylor 2005a ).
In conclusion, despite two and a half centuries of investigation, the general picture of cheilostome brood-chamber
structure and development remains incomplete. The largest
single published source of information is Levinsen’s ( 1909 ).
2.3 Structure and Development of Brood Chambers in Cheilostomata
with the operculum above it. The distal margin of the operculum is situated close to the proximal border of the ovicell
but does not adjoin it.
The ovicells of Thalamoporella are distinctive. Levinsen
( 1902 , p. 15) referred to them as “epistomial” but later
considered them to be hyperstomial (Levinsen 1909 ).
Harmer ( 1926 , p. 291) suggested that they are non-homologous to hyperstomial ovicells in other Cheilostomata, proposing that they evolved from the “adoral tubercles” of the
maternal zooid. Marcus ( 1941a , pl. 4, fi g. 11) presented the
stages of ovicellogenesis and a schematic of a longitudinal
section of the ovicelled autozooid of T . evelinae . It gives the
impression that the ooecium consists of three walls in this
species. Marcus did not show any communication organs
between ooecial and zooidal coeloms.
My study of Thalamoporella sp. showed that the ooecium of the cleithral ovicell is formed from the maternal
autozooid, which has a larger orifi ce than non-ovicelled
zooids (Fig. 2.49A, D ). This is a special type of bivalved
ovicell, formed at the frontal surface of the maternal autozooid around its orifi ce (see also Levinsen 1902 ; Harmer
1926 ). The intermediate stage of ovicellogenesis superfi -
cially resembles ooecial-fold development in calloporids
(Fig. 2.49D ). The calcifi ed ooecium results from the fusion
of two symmetrical, hollow hemispherical lobes along the
midline of the ooecium, leaving a medial suture visible
externally and inter nally (Fig. 2.49B, C, E, F , see also pl. 4,
fi g. 7 in Marcus 1941a ). In contrast with the calloporid
Bryocalyx cinnameus , in which the ooecial lobes are separated by a double longitudinal septum in the distal part of
the ooecium, the lobes in Thalamoporella are separated by a
septum only at the ooecial base (Fig. 2.49E ); in the upper
part there is no septum and the ooecial roof is thus complete
even though the medial suture is retained (Fig. 2.49F ).
Waters ( 1909 , p. 142) termed the ovicells of Thalamoporella
“bilobate,” but also stated that “there is no complete divisional wall in” them. The nature of the internal “wall” in the
ooecium seen in the above- mentioned illustration of Marcus
is puzzling since it was not shown in another of Marcus’s
fi gures ( 1941a , pl. 5, fi g. 12a).
The ooecial coelom communicates directly with the
visceral coelom of the maternal autozooid via two large,
symmetrical arched openings at the sides of its aperture
(Fig. 2.49C ). Thus, in this case, although the ooecium is
formed at the expense of the maternal autozooid, it is not a
kenozooid but a paired outgrowth of the frontal zooidal wall.
2.3.5 Acanthostegal Brood Chambers
These structures, made of fl attened mural spines, are known
only in three living species of Tendridae (Cheilostomata)
(Hincks 1892 ; Levinsen 1909 ; Ostrovsky and Taylor
2005a ). Repiachoff ( 1875 ), Reinhard ( 1875 ) and Ostroumoff
( 1886a , b , c ) studied them in Tendra zostericola . Although
mistaken in their understanding of the construction of these
brood chambers, and believing that embryos were developed inside the body cavity of specialized zooids, Repiahoff
( 1875 ) nevertheless suggested that they play the role of ovicells, and Reinhard ( 1875 , p. 25) stated that “ Tendra will
represent a transition between bryozoans without ovicells
to those that possess them”. Ostroumoff ( 1886a ) was the
fi rst to understand that the embryos are brooded in the
space [epistege] between the frontal membrane and the
over-arching spines in this species (see also Appendix I for
historical review).
In Tendra zostericola , the brooding zooid produces a pair
of articulated oral spines and, at the mural edge, two (sometimes one) lateral rows of horizontally inclined inarticulate
spines that are fl attened at the base. These long, pointed
spines closely adjoin each other and the spines of the opposite row, forming the acanthostegal (literally “spine-roofed”)
brood chamber; the space between it and the underlying
frontal membrane is the brood cavity (Figs. 2.50 and 2.59A ).
Each lateral row typically consists of 10–15 spines (up to 17
(Repiachoff 1875 ; Levinsen 1909 ), 13–18 (Occipinti Ambrogi
1981 ; Occhipinti Ambrogi and d’Hondt 1981 )). The proximal edge is free of spines, providing an opening for oviposition and larval release (Fig. 2.50B ). It may remain open but
is usually closed by the operculum of the proximal (maternal)
autozooid, as in the case of the cleithral ovicells of other
cheilostomes.
The so-called brooding “zooids” of Heteroecium amplectens are a complex of two zooids – the proximal (maternal)
autozooid (apparently an autozooidal polymorph) and a distal kenozooid (Figs. 2.51A, B and 2.59B ). At the mural edge
of the latter, up to 15–17 fl attened inarticulate spines form
the roof of the brood chamber, similar to the situation in
Tendra . They closely adjoin each other, leaving no spaces
between, their ends fusing along the midline of the kenozooid to form a low longitudinal keel. The brood chamber has
the shape of an elongated hemisphere with a single proximal
opening closed by the operculum of the maternal zooid, similar to cleithral closure in other cheilostomes. The broodcavity fl oor is calcifi ed, except for a proximal membranous
area where there are two lateral outgrowths (Fig. 2.51C, D
and 2.59B ) facing the kenozooidal coelomic cavity. It may
be conjectured that this area of kenozooidal frontal wall
is a rudiment of the frontal membrane of the autozooid,
with parietal musculature. The lateral outgrowths would
then serve for attachment of these muscles (Ostrovsky and
Taylor 2005a ).
In conclusion, despite two and a half centuries of investigation, the general picture of cheilostome brood-chamber
structure and development remains incomplete. The largest
single published source of information is Levinsen’s ( 1909 ).
2.3 Structure and Development of Brood Chambers in Cheilostomata