Deuterostomes develop an anus before any other openings. At some point, you were nothing but an asshole… and some people never develop beyond this stage.

Deuterostomia (; lit. ’second mouth’ in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group’s sister clade is Protostomia, animals whose digestive tract development is more varied. Some examples of Deuterostomes include vertebrates (and thus humans), sea stars, and crinoids.

In deuterostomy, the developing embryo’s first opening (the blastopore) becomes the anus, while the mouth is formed at a different site later on. This was initially the group’s distinguishing characteristic, but deuterostomy has since been discovered among protostomes as well. This group is also known as enterocoelomates, because their coelom develops through enterocoely.

The three major clades of deuterostomes are Chordata (e.g. vertebrates), Echinodermata (e.g. starfish), and Hemichordata (e.g. acorn worms). Together with Protostomia and their out-group Xenacoelomorpha, these compose the Bilateria, animals with bilateral symmetry and three germ layers.


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