301
having gonochoristic zooids. In contrast with male and
sterile zooids, females are characterized by a smaller polypide with fewer tentacles, a very large intertentacular organ
and two opercula, separately closing the ovicell and the zooidal orifi ce. These were described fi rst by Levinsen ( 1909 )
(see also Harmer ( 1926 ) and Hastings ( 1930 )). Spermatozoids
united in pairs (“twin sperm”, p. 142), presumably exiting
the male zooid via the coelomopore and entering the female
coelom through the intertentacular organ. Insemination is
intraovarian and monospermic. The ovary develops from the
peritoneal cells of the basal cystid wall in the distal part of
the female zooid. A follicle from the squamose cells envelops the growing oocyte situated on top of the pedunculate
part (“ovarian stalk”), the central area of which Marcus compared to a “nutritive channel” (p. 41). As seen from his Plate
6, fi g. 16, the pedunculate part consists of a large subovarian
space with young oocyte doublets on its periphery and containing alien sperm. Marcus was the fi rst to note that oocytes
develop in pairs (he depicts an ovary with up to four doublets), in which one of the cells plays the role of nurse.
Actually, he saw and depicted this phenomenon in his 1938a
paper also, but did not then describe it. According to his
description, in Thalamoporella evelinae two young oocytes
fuse when both reach 20–30 μm diameter (in all other known
instances, cheilostome oocyte doublets are the result of
arrested cytokinesis). Shortly after this fusion, syngamy
takes place with one of the oocytes [that will become an
egg], whereas the second cell becomes a nurse (“cellula auxiliar”, p. 36). The doublet grows, and when it reaches its fi nal
size, the nucleus of the nurse cell migrates through the cytoplasm to the vegetal pole where it is expelled, with the nurse
cell becoming incorporated into the oocyte. Marcus also
wrote that the oocyte could be nourished on account of the
“yolk stored in the peritoneal cells” of the lateral body wall
(p. 142), and by the special area of “high [hypertrophied]
peritoneal epithelium on the front wall”, close to or adjoining
the leading oocyte. Cells of the ovary (including follicle cells)
accumulate yolk (provided by peritoneal storage cells), which
is further transported to the oocyte “with the help of ovarian
stroma”. Up to six embryos of different ages were recorded
as being brooded in the ovicell. Additionally, Marcus described
an intertentacular organ in Alcyonidium polypylum ; in this
species the ovary is proximal, whereas spermatogenic tissue
occurs proximally and distally in hermaphrodite zooids.
In the same year, Marcus ( 1941b ) published a paper on
the cheilostome Synnotum sp. (as S . aegyptiacum ), in which
he discovered intracoelomic embryonic incubation (viviparity). In this species, different gonads appear simultaneously
in paired gonochoristic zooids. The ovary in this case produces 2–3 oocytes, one of which develops into an embryo
inside the maternal zooid whose polypide degenerates. The
embryo “is nourished by the follicle cells which receive alimentary material from other parts of the colony and the
maternal brown body, transported by the mesenchymatous
tissue-cords” (p. 232). The late embryo is 50–60 times larger
than the mature ovum – good evidence of extraembryonic
nutrition.
Cori ( 1941 ) reviewed bryozoan sexual reproduction in his
textbook. In general, he characterized this complex phenomenon correctly, except that polyspermy and self-fertilization
were considered to be common to all Bryozoa and the ooecium was said to develop as part of the maternal zooid. It
should be noted that, studying Zoobotryon verticillatum (as
Z . pellucidum ), Cori found and depicted spermatozoids in
the coelomic lumen of the tentacles. Later Brien ( 1960 ) mentioned this, suggesting that sperm is released via the terminal
tentacular pores. Cori presented one of the most comprehensive lists of bryozoan literature for this period.
Silén ( 1944 , p. 18) investigated the ctenostome
Labiostomella gisleni (as a cheilostome), recording more
than 100 oocytes simultaneously in its very long ovary surrounded by a “very thin … fi lm [of] fl attened cells” [ovary
wall]. However, he came to the conclusion that only one
embryo is developed during the “breeding season” of each
fertile zooid. Zooids were stated to be hermaphrodite and
protogynous. Silén described different stages of oocyte
growth, measured them, discussed ovulation and fertilization, and noted that ovulated oocytes accumulate in the distal
part of the autozooid. He also noted the presence of a male
nucleus inside them. Since no ripe spermatozoids were found
in testes, he suggested that the sperm came from outside, fusing with the eggs in the distal region of the zooid. One
embryo per zooid developed inside an embryo sac [presumably
possessing an embryophore, see Silén 1944 , text-fi g. 11],
accompanied by polypide degeneration. Silén admitted that
the sac is an invagination of the body wall, but speculated
that its formation is strongly modifi ed, developing by inward
migration of ectodermal cells that overgrow and envelop the
fertilized egg. In comparing this embryo sac with that found
in the ctenostomes Sundanella and Nolella , he concluded
that they are homologous, having the same type of development. Larval release occurred through rupture of the internal
wall of the sac and further via the zooid aperture. Based on
these fi ndings and ovicellar anatomy of the cheilostome
Scrupocellaria scabra , Silén proposed a hypothesis suggesting that cheilostome ovicells originated from an embryo sac
like that in Labiostomella (see also Ström 1977 ). He also
considered brooding structures throughout the phylum to be
homologous, originating from a modifi ed polypide. The
cyphonautes larva was stated to be a derived larval type (see
Chap. 3 of the main text).
Several years later Braem ( 1951 ), investigating the
ctenostomes Bulbella abscondita and Victorella muelleri
(as Tanganella ), showed that oviposited zygotes stick
either to the everted vestibulum. In Bulbella , the ovary is positioned on the cystid wall in the middle region of the zooid.
Appendices
having gonochoristic zooids. In contrast with male and
sterile zooids, females are characterized by a smaller polypide with fewer tentacles, a very large intertentacular organ
and two opercula, separately closing the ovicell and the zooidal orifi ce. These were described fi rst by Levinsen ( 1909 )
(see also Harmer ( 1926 ) and Hastings ( 1930 )). Spermatozoids
united in pairs (“twin sperm”, p. 142), presumably exiting
the male zooid via the coelomopore and entering the female
coelom through the intertentacular organ. Insemination is
intraovarian and monospermic. The ovary develops from the
peritoneal cells of the basal cystid wall in the distal part of
the female zooid. A follicle from the squamose cells envelops the growing oocyte situated on top of the pedunculate
part (“ovarian stalk”), the central area of which Marcus compared to a “nutritive channel” (p. 41). As seen from his Plate
6, fi g. 16, the pedunculate part consists of a large subovarian
space with young oocyte doublets on its periphery and containing alien sperm. Marcus was the fi rst to note that oocytes
develop in pairs (he depicts an ovary with up to four doublets), in which one of the cells plays the role of nurse.
Actually, he saw and depicted this phenomenon in his 1938a
paper also, but did not then describe it. According to his
description, in Thalamoporella evelinae two young oocytes
fuse when both reach 20–30 μm diameter (in all other known
instances, cheilostome oocyte doublets are the result of
arrested cytokinesis). Shortly after this fusion, syngamy
takes place with one of the oocytes [that will become an
egg], whereas the second cell becomes a nurse (“cellula auxiliar”, p. 36). The doublet grows, and when it reaches its fi nal
size, the nucleus of the nurse cell migrates through the cytoplasm to the vegetal pole where it is expelled, with the nurse
cell becoming incorporated into the oocyte. Marcus also
wrote that the oocyte could be nourished on account of the
“yolk stored in the peritoneal cells” of the lateral body wall
(p. 142), and by the special area of “high [hypertrophied]
peritoneal epithelium on the front wall”, close to or adjoining
the leading oocyte. Cells of the ovary (including follicle cells)
accumulate yolk (provided by peritoneal storage cells), which
is further transported to the oocyte “with the help of ovarian
stroma”. Up to six embryos of different ages were recorded
as being brooded in the ovicell. Additionally, Marcus described
an intertentacular organ in Alcyonidium polypylum ; in this
species the ovary is proximal, whereas spermatogenic tissue
occurs proximally and distally in hermaphrodite zooids.
In the same year, Marcus ( 1941b ) published a paper on
the cheilostome Synnotum sp. (as S . aegyptiacum ), in which
he discovered intracoelomic embryonic incubation (viviparity). In this species, different gonads appear simultaneously
in paired gonochoristic zooids. The ovary in this case produces 2–3 oocytes, one of which develops into an embryo
inside the maternal zooid whose polypide degenerates. The
embryo “is nourished by the follicle cells which receive alimentary material from other parts of the colony and the
maternal brown body, transported by the mesenchymatous
tissue-cords” (p. 232). The late embryo is 50–60 times larger
than the mature ovum – good evidence of extraembryonic
nutrition.
Cori ( 1941 ) reviewed bryozoan sexual reproduction in his
textbook. In general, he characterized this complex phenomenon correctly, except that polyspermy and self-fertilization
were considered to be common to all Bryozoa and the ooecium was said to develop as part of the maternal zooid. It
should be noted that, studying Zoobotryon verticillatum (as
Z . pellucidum ), Cori found and depicted spermatozoids in
the coelomic lumen of the tentacles. Later Brien ( 1960 ) mentioned this, suggesting that sperm is released via the terminal
tentacular pores. Cori presented one of the most comprehensive lists of bryozoan literature for this period.
Silén ( 1944 , p. 18) investigated the ctenostome
Labiostomella gisleni (as a cheilostome), recording more
than 100 oocytes simultaneously in its very long ovary surrounded by a “very thin … fi lm [of] fl attened cells” [ovary
wall]. However, he came to the conclusion that only one
embryo is developed during the “breeding season” of each
fertile zooid. Zooids were stated to be hermaphrodite and
protogynous. Silén described different stages of oocyte
growth, measured them, discussed ovulation and fertilization, and noted that ovulated oocytes accumulate in the distal
part of the autozooid. He also noted the presence of a male
nucleus inside them. Since no ripe spermatozoids were found
in testes, he suggested that the sperm came from outside, fusing with the eggs in the distal region of the zooid. One
embryo per zooid developed inside an embryo sac [presumably
possessing an embryophore, see Silén 1944 , text-fi g. 11],
accompanied by polypide degeneration. Silén admitted that
the sac is an invagination of the body wall, but speculated
that its formation is strongly modifi ed, developing by inward
migration of ectodermal cells that overgrow and envelop the
fertilized egg. In comparing this embryo sac with that found
in the ctenostomes Sundanella and Nolella , he concluded
that they are homologous, having the same type of development. Larval release occurred through rupture of the internal
wall of the sac and further via the zooid aperture. Based on
these fi ndings and ovicellar anatomy of the cheilostome
Scrupocellaria scabra , Silén proposed a hypothesis suggesting that cheilostome ovicells originated from an embryo sac
like that in Labiostomella (see also Ström 1977 ). He also
considered brooding structures throughout the phylum to be
homologous, originating from a modifi ed polypide. The
cyphonautes larva was stated to be a derived larval type (see
Chap. 3 of the main text).
Several years later Braem ( 1951 ), investigating the
ctenostomes Bulbella abscondita and Victorella muelleri
(as Tanganella ), showed that oviposited zygotes stick
either to the everted vestibulum. In Bulbella , the ovary is positioned on the cystid wall in the middle region of the zooid.
Appendices