protostome
(noun)
any animal in which the mouth is derived first from the embryonic blastopore ("mouth first")
Examples of protostome in the following topics:
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Superphylum Lophotrochozoa
- The Lophotrochozoa are protostomes possessing a blastopore, an early form of a mouth; they include the trochozoans and the lophophorata.
- Animals belonging to superphylum Lophotrochozoa are protostomes: the blastopore (or the point of involution of the ectoderm or outer germ layer) becomes the mouth opening to the alimentary canal.
- This layer multiplies into a band which then splits internally to form the coelom; this protostomic coelom is termed schizocoelom.
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Animal Characterization Based on Features of Embryological Development
- These two groups are separated based on which opening of the digestive cavity develops first: mouth (protostomes) or anus (deuterostomes) .
- The word protostome comes from the Greek word meaning "mouth first. " The protostomes include arthropods, mollusks, and annelids.
- Protostomes also undergo determinate cleavage: the developmental fate of each embryonic cell is pre-determined.
- Unlike protostomes, deuterostomes undergo indeterminate cleavage: cells remain undifferentiated until a later developmental stage.
- In protostomes, part of the mesoderm separates to form the coelom in a process called schizocoely.
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Molecular Analyses and Modern Phylogenetic Trees
- Extensive molecular analysis using rRNA data found these animals to be protostomes, more closely related to annelids and mollusks.
- This discovery allowed for the distinction of the protostome clade: the lophotrochozoans.
- Some scientists believe that the phyla Platyhelminthes and Rotifera within this group should actually belong to their own group of protostomes termed Platyzoa.
- These two clades now make up the protostomes.
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Constructing an Animal Phylogenetic Tree
- The bilaterally-symmetrical animals are further divided into deuterostomes (including chordates and echinoderms) and two distinct clades of protostomes (including ecdysozoans and lophotrochozoans).
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Phylum Annelida
- Annelids show protostomic development in embryonic stages and are often called "segmented worms" due to their key characteristic of metamerism, or true segmentation.
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Phylum Arthropoda
- Arthropods are eucoelomate, protostomic organisms.
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Characteristics of Vertebrates
- Animals that possess bilateral symmetry can be divided into two groups, protostomes and deuterostomes, based on their patterns of embryonic development.