Inversion of the chordate body axis: are there alternatives?
One major morphological difference between chordates and annelids or arthropods is the opposite orientation of the nerve cord and heart. A long-standing proposal is that the chordate axis evolved by inverting the body of an ancestor with the annelid/arthropod orientation. However, the data can also be explained by a common ancestor with diffuse dorsoventral organization, followed by oppositely directed condensation of the nerve cord and relocation of the heart in the two lines.
Gerhart
J
J
Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720-3200, USA. gerhart@socrates.berkeley.edu
D000818:Animals; D000845:Annelida / Q000033:anatomy & histology / Q000196:embryology; D001181:Arthropods / Q000033:anatomy & histology / Q000196:embryology; D019521:Body Patterning; D002816:Chordata, Nonvertebrate / Q000033:anatomy & histology* / Q000196:embryology / Q000235:genetics; D019143:Evolution, Molecular; D010802:Phylogeny; D012337:RNA, Ribosomal, 18S / Q000235:genetics
D012337:RNA, Ribosomal, 18S