An animal in which the anus develops from the first opening (blastopore) formed during embryonic development, while the mouth forms secondarily. This group includes vertebrates, echinoderms, and some other phyla.
From Greek 'deuteros' (second) and 'stoma' (mouth), literally meaning 'second mouth.' The term was coined in the early 20th century by German embryologist Karl Grobben based on observations of embryonic development patterns.
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