The cnidarian premises of metazoan evolution: From triploblasty, to coelom formation, to metamery
Abstract
The hydromedusan subumbrellar muscle tissues originate from a mass of endo‐ and ectoderm derived cells proliferating inwardly. This mass of cells, called entocodon, is separated by the ecto‐and endoderm through a layer of extracellular matrix, thus forming a locally triploblastic arrangement of tissues. By cavitation and differentiation, the entocodon gives rise to the striated and smooth muscle layers of the subumbrella. The structure of the striated muscle is histologically identical to that described for triploblasts, where striated muscle is mainly mesodermic. Together with the mode of development, this suggests that not all Cnidaria are diploblastic and that the subumbrellar cavity is a coelom‐like structure. The subumbreHar cavity is formed late in ontogeny, whereas the coelom in higher animals is normally formed during embryonic development. Instead of remaining closed, the subumbrella becomes open, with the muscular mesothelium in contact with the environment. This view of cnidarian structures can be pushed further while considering the regulation of modularity and metamery. The occurrence of homeotic genes in cnidarians and other ‘lower’ animals shows that the specification of body architecture has a common molecular ground throughout the animal kingdom. It is suggested that the expression of homeotic genes while specifying a given architecture might be regulated by iteration of their activity, leading to ‘low’ modular organisms and to ‘higher’ metameric ones. Replicated parts as modules and metameres might be analogous, but the affinity in their molecular specification (and possibly regulation) might indicate homology. The number of resemblances between cnidarians and ‘higher’ metazoa suggests that this phylum might be a centre of metazoan radiation, having structures which represent the living premises of the main steps of animal evolution.
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The entocodon qualifies as a third germ layer because its cells are separated from ectoderm and endoderm by an extracellular matrix.
“Because the entocodon cells are clearly separated from the ectoderm and endoderm by the formation of an extracellular matrix (Bo ¨lsterli, 1977) the entocodon qualifies per definition as a third germ layer”
From Evolution of striated muscle: Jellyfish and the origin of triploblasty (2005) · cites this paper as Boero et al., 1998
These observations support the hypothesis that the striated muscle of jellyfish is related to the striated muscle of bilaterians.
“These observations support the hypothesis (Boero et al., 1998;Spring et al., 2000Spring et al., , 2002) ) that the striated muscle of jellyfish is related to the striated muscle of bilaterians.”
From Evolutionary aspects of developmentally regulated helix-loop-helix transcription factors in striated muscle of jellyfish (2003) · cites this paper as Boero et al., 1998
Because entocodon cells are clearly separated from ectoderm and endoderm by an extracellular matrix, they constitute a third germ layer.
“Because the entocodon cells are clearly separated from the ectoderm and endoderm by the formation of an extracellular matrix (Boelsterli, 1977; m in Fig. 5), they constitute a third germ layer”
From Evolutionary aspects of developmentally regulated helix-loop-helix transcription factors in striated muscle of jellyfish (2003) · cites this paper as Boero et al., 1998
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- Authors
- Boero, F., Gravili, C., Pagliara, P., Piraino, S., Bouillon, J., & Schmid, V.
- Affiliations
- University of Salento (IT)
- b Istituto Sperimentale Talassografico “Attilio Cerruti”; del , CNR , via Roma 4, Taranto, I‐74100, Italy
- Université Libre de Bruxelles (BE)
- University of Basel (CH)
- Venue
- Italian Journal of Zoology
- Publisher
- Informa UK Limited
- Published
- 1998-01
- Volume
- 65
- Issue
- 1
- Pages
- 5-9
- ISSN
- 1125-0003 (ISSN-L)
- 1748-5851
- OpenAlex
- W1967695370
- Cited by
- 77
- References
- 15
- Citations by year
- 2025: 1 · 2024: 1 · 2020: 1 · 2019: 3 · 2018: 2 · 2017: 1 · 2015: 3 · 2012: 2
- FWCI
- 2.82
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- The European Zoological Journal (publishedVersion)
- Dépôt institutionnel de l'Université libre de Bruxelles (Université Libre de Bruxelles) (submittedVersion)
- AIRUS (submittedVersion)
- Topics
- Physical Sciences › Earth and Planetary Sciences › Paleontology › Marine Invertebrate Physiology and Ecology
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Developmental Biology and Gene Regulation
- Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Planarian Biology and Electrostimulation
- Keywords
- Cnidaria, animal evolution, hydromedusae, striated muscle, homeotic genes, modularity, body structure, homology, mesothelium, cnidarian phylogeny
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