Concept
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Chordates and the Evolution of Vertebrates
Cephalochrodates
The lancelet, like all cephalochordates, has a head. Adult lancelets retain the four key features of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. Water from the mouth enters the pharyngeal slits, which filter out food particles. The filtered water then collects in the atrium and exits through the atriopore.
Source
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"OpenStax College, Chordates. October 17, 2013."
http://cnx.org/content/m44685/latest/Figure_29_01_04.jpg
OpenStax CNX
CC BY 3.0.