from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A multicellular animal of the subkingdom Metazoa, a division of the animal kingdom in traditional two-kingdom classification systems.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Any animal that undergoes development from an embryo stage with three tissue layers, namely the ectoderm, mesoderm, and endoderm. The term applies to all animals except the sponges.
- adj. Having to do with animals that develop from an embryo with three tissue layers.
from the GNU version of the Collaborative International Dictionary of English
- n. One of the Metazoa.
from The Century Dictionary and Cyclopedia
- Of or pertaining to the Metazoa.
- n. A member of the Metazoa; a metazoön.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. any animal of the subkingdom Metazoa; all animals except protozoans and sponges
Consider two classes of transcription factors that are known to play crucial roles in metazoan development: the T-box proteins and the SOX proteins.
These proteins play essential roles in metazoan development and are considered part of the developmental toolkit as outlined by biologist Sean Carroll.
He briefly discusses a recent paper where "The authors concluded that gene loss had been a substantial factor in evolution, and that" the ancestral metazoan is likely to have been much more complex than was previously imagined. "
They are just the simplest, barest kind of metazoan we can find now.
I think this is very subjective; I was quite happy with Merriam Webster's definition of "metazoan," for example.
A data dump now about problems with metazoan (or chordate or mammalian) phylogeny would be getting ahead of the discussion.
If I were a FLID supporter, I would propose that the homeobox regulatory mechanism for body regions was a "front-loaded design element" that, once it was in place, could be modified extensively via standard evolutionary mechanisms to produce the various metameric metazoan bauplans.
Even these points are disputable due to their metazoan-centric nature.
There is no compelling evidence that the lineage that led to complex, metazoan life forms (including humans) and the lineage that led to the single-celled Monosiga independently experienced significantly similar niches or were exposed to similar environments.
Guts: There is no compelling evidence that the lineage that led to complex, metazoan life forms (including humans) and the lineage that led to the single-celled Monosiga independently experienced significantly similar niches or were exposed to similar environments.
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