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The sicula is the initial theca in the graptolite colony, i.e., it is
interpreted as a skeleton of initial zooid. It is known to
consist of two parts called the prosicula (secreted as single
unit) and metasicula (composed of normal fusellar growth
bands).
Roman Kozłowski (1949, pp. 98-101), supposed that the
prosicula was the product of an oozooid, from which the
metazooid responsible for building up the metasicula evolved.
Roman Kozłowski (1971) distinguished two types of siculae:
discophorous and nematophorous. Sessile graptolites have
disciphorous siculae, resembling embryonal vesicle of
Rhabdopleura. The discophorous sicula has a basal disc, a mean
of attachment to the substrate. Undoubtedly, they represent
ancestral morphoecologic type of organization.
Nematophorous siculae are characteristic for free-living
graptolites - in these siculae the basal disc is replaced
secondarily by a threadlike extension of apex of prosicula
called nema.
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interpreted as a skeleton of initial zooid. It is known to
consist of two parts called the prosicula (secreted as single
unit) and metasicula (composed of normal fusellar growth
bands).
Roman Kozłowski (1949, pp. 98-101), supposed that the
prosicula was the product of an oozooid, from which the
metazooid responsible for building up the metasicula evolved.
Roman Kozłowski (1971) distinguished two types of siculae:
discophorous and nematophorous. Sessile graptolites have
disciphorous siculae, resembling embryonal vesicle of
Rhabdopleura. The discophorous sicula has a basal disc, a mean
of attachment to the substrate. Undoubtedly, they represent
ancestral morphoecologic type of organization.
Nematophorous siculae are characteristic for free-living
graptolites - in these siculae the basal disc is replaced
secondarily by a threadlike extension of apex of prosicula
called nema.
___________________________________
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