119
the coelomic cavity of the maternal zooid and considered it
part of the latter.
The comprehensive studies of Levinsen ( 1893 , 1894 ,
1902 , 1909 , 1916 , 1925 ) (who intentionally did not use the
term “ovicell,” possibly because it refl ected the erroneous
idea that eggs can be formed in them), revealed “numerous
modifi cations” of “hyperstomial ooecia” and showed a basic
similarity in their structure, where “the two layers [walls] of
the actual ooecium are formed by the frontal membrane [wall]
of the distal zooecium [daughter zooid]” (Levinsen 1909 ,
p. 60). He also described and depicted some species with
ooecia formed either by distal kenozooids or avicularia, but
stated that the above-mentioned “type of the ooecium …
appears in the majority of the Cheilostomata”. Likewise, in
considering endozooidal ovicells, he categorized them into
“ooecia which are enclosed in autozooecia” and “ooecia which
are surrounded by kenozooecia” (Levinsen 1909 , pp. 56, 59).
He did not mention the communication between ooecial
and distal zooidal coeloms discovered by Calvet ( 1900 )
in Bugula , or depict a communication pore in his schema
of the ovicell of Bugula (Levinsen 1909 , pl. 24, fi g. 13).
However, he carefully illustrated it in many other cases
(Levinsen, 1893 , 1894 , 1909 ). One explanation may be that
Levinsen mainly dealt with cleaned (but often sectioned)
skeletons in which communication pores are not always
clearly visible.
In total, Levinsen described ovicell structure and development in more than 80 cheilostome species, but, except for
his terminology, his data were practically never used (see
Ostrovsky 2008a , b ). He classifi ed cheilostome brood
chambers according to their structure, the position of the
ooecium relative to the zooidal orifi ce, and degree of ovicell
immersion, introducing the terms “endozooecial”, “hyperstomial”, “peristomial”, “endotoichal”, “double-valved” and
“acanthostegous,” most of which are currently in use
(Levinsen 1902 , 1909 ). He also categorized hyperstomial
ovicells as (1) “ooecia without a cryptocyst” and “ooecia
with a cryptocyst” (Levinsen 1902 ), and (2) “dependent” and
“independent” according to the number of ooecial walls and
the size of the contact between the ooecial base and the
distal zooid wall (Levinsen 1909 ). He often used the terms
“ooecial fold” for the entire ooecium, “ooecial operculum” for
the ooecial vesicle, and “ectoOoecium” and “endoOoecium”
for the external and ‘internal’ [surrounding a brood cavity]
ooecial walls (Levinsen 1902 , p. 13, 1909 , p. 60). He also
described the earliest stages of ovicellogenesis (in dried
specimens) which, according to him, start from the development of either “two small distal calcareous plates” or “a
continuous plate” (depending on the taxon), arising “from
the frontal edge of the distal [zooidal] wall” (Levinsen 1909 ,
pp. 60–61; see also Ostrovsky and Taylor 2005a ). In the
same monograph he suggested that the egg should leave
the maternal zooid before entering the ovicell, aided by the
tentacle sheath as suggested by Jullien ( 1888 ) in Celleporella
hyalina (as Hippothoa ) or “by an independent movement of
the egg” (p. 67).
Subsequent authors either accepted without discussion,
or supported, or just ignored the findings of previous
workers on ooecial structure. Korschelt and Heider ( 1910 )
briefl y described ovicell structure in Bugula subsequent to
Calvet and copied the schema of the ovicell in sagittal section from his monograph (Calvet 1900 , fi g. 10) without comment. Canu and Bassler ( 1920 ), although criticizing
Levinsen ( 1902 , 1909 ), gave very similar schemata of
different ovicellar types (see also Bassler 1922 , 1953 ).
These authors sectioned a number of species with ovicells
and introduced the term “subcleithriens” for cheilostomes
with ovicells closed by the partly elevated operculum. Canu
and Bassler ( 1920 ) substituted Levinsen’s term “independent ooecia” for “recumbent” [Ryland ( 1968 ) criticized
this move] and reproduced Calvet’s schema for the Bugula
ovicell (see Canu and Bassler 1929 ). These authors also
applied the characters of ovicell structure (immersion and
closure) to the classifi cation of “Membraniporae” (Canu
and Bassler 1923 ).
Fig. 2.3 Calvet’s ( 1900 ) depiction of ovicell structure in Bugula
simplex . Abbreviations: bi coelom of maternal autozooid, bs coelom of
daughter autozooid, cu cuticle, e embryo, eiv brood cavity, ep epidermis, epi oral spine, fu funicular strands, gt wall of tentacular sheath,
mud muscles-depressors of brooding cavity, mur muscles-retractors of
ooecial vesicle, o communication between the cavity of ooecial vesicle
and the cavity of maternal zooid, oz zooidal orifi ce, pc communication
pore between the coelomic cavity of ooecium and the visceral coelom
of the distal zooid, pzf frontal wall of maternal autozooid, voi ooecial
vesicle cavity, vos ooecial coelomic cavity
2.1 History of Studies of Cheilostome Brood Chambers
the coelomic cavity of the maternal zooid and considered it
part of the latter.
The comprehensive studies of Levinsen ( 1893 , 1894 ,
1902 , 1909 , 1916 , 1925 ) (who intentionally did not use the
term “ovicell,” possibly because it refl ected the erroneous
idea that eggs can be formed in them), revealed “numerous
modifi cations” of “hyperstomial ooecia” and showed a basic
similarity in their structure, where “the two layers [walls] of
the actual ooecium are formed by the frontal membrane [wall]
of the distal zooecium [daughter zooid]” (Levinsen 1909 ,
p. 60). He also described and depicted some species with
ooecia formed either by distal kenozooids or avicularia, but
stated that the above-mentioned “type of the ooecium …
appears in the majority of the Cheilostomata”. Likewise, in
considering endozooidal ovicells, he categorized them into
“ooecia which are enclosed in autozooecia” and “ooecia which
are surrounded by kenozooecia” (Levinsen 1909 , pp. 56, 59).
He did not mention the communication between ooecial
and distal zooidal coeloms discovered by Calvet ( 1900 )
in Bugula , or depict a communication pore in his schema
of the ovicell of Bugula (Levinsen 1909 , pl. 24, fi g. 13).
However, he carefully illustrated it in many other cases
(Levinsen, 1893 , 1894 , 1909 ). One explanation may be that
Levinsen mainly dealt with cleaned (but often sectioned)
skeletons in which communication pores are not always
clearly visible.
In total, Levinsen described ovicell structure and development in more than 80 cheilostome species, but, except for
his terminology, his data were practically never used (see
Ostrovsky 2008a , b ). He classifi ed cheilostome brood
chambers according to their structure, the position of the
ooecium relative to the zooidal orifi ce, and degree of ovicell
immersion, introducing the terms “endozooecial”, “hyperstomial”, “peristomial”, “endotoichal”, “double-valved” and
“acanthostegous,” most of which are currently in use
(Levinsen 1902 , 1909 ). He also categorized hyperstomial
ovicells as (1) “ooecia without a cryptocyst” and “ooecia
with a cryptocyst” (Levinsen 1902 ), and (2) “dependent” and
“independent” according to the number of ooecial walls and
the size of the contact between the ooecial base and the
distal zooid wall (Levinsen 1909 ). He often used the terms
“ooecial fold” for the entire ooecium, “ooecial operculum” for
the ooecial vesicle, and “ectoOoecium” and “endoOoecium”
for the external and ‘internal’ [surrounding a brood cavity]
ooecial walls (Levinsen 1902 , p. 13, 1909 , p. 60). He also
described the earliest stages of ovicellogenesis (in dried
specimens) which, according to him, start from the development of either “two small distal calcareous plates” or “a
continuous plate” (depending on the taxon), arising “from
the frontal edge of the distal [zooidal] wall” (Levinsen 1909 ,
pp. 60–61; see also Ostrovsky and Taylor 2005a ). In the
same monograph he suggested that the egg should leave
the maternal zooid before entering the ovicell, aided by the
tentacle sheath as suggested by Jullien ( 1888 ) in Celleporella
hyalina (as Hippothoa ) or “by an independent movement of
the egg” (p. 67).
Subsequent authors either accepted without discussion,
or supported, or just ignored the findings of previous
workers on ooecial structure. Korschelt and Heider ( 1910 )
briefl y described ovicell structure in Bugula subsequent to
Calvet and copied the schema of the ovicell in sagittal section from his monograph (Calvet 1900 , fi g. 10) without comment. Canu and Bassler ( 1920 ), although criticizing
Levinsen ( 1902 , 1909 ), gave very similar schemata of
different ovicellar types (see also Bassler 1922 , 1953 ).
These authors sectioned a number of species with ovicells
and introduced the term “subcleithriens” for cheilostomes
with ovicells closed by the partly elevated operculum. Canu
and Bassler ( 1920 ) substituted Levinsen’s term “independent ooecia” for “recumbent” [Ryland ( 1968 ) criticized
this move] and reproduced Calvet’s schema for the Bugula
ovicell (see Canu and Bassler 1929 ). These authors also
applied the characters of ovicell structure (immersion and
closure) to the classifi cation of “Membraniporae” (Canu
and Bassler 1923 ).
Fig. 2.3 Calvet’s ( 1900 ) depiction of ovicell structure in Bugula
simplex . Abbreviations: bi coelom of maternal autozooid, bs coelom of
daughter autozooid, cu cuticle, e embryo, eiv brood cavity, ep epidermis, epi oral spine, fu funicular strands, gt wall of tentacular sheath,
mud muscles-depressors of brooding cavity, mur muscles-retractors of
ooecial vesicle, o communication between the cavity of ooecial vesicle
and the cavity of maternal zooid, oz zooidal orifi ce, pc communication
pore between the coelomic cavity of ooecium and the visceral coelom
of the distal zooid, pzf frontal wall of maternal autozooid, voi ooecial
vesicle cavity, vos ooecial coelomic cavity
2.1 History of Studies of Cheilostome Brood Chambers