Phylum CHORDATA, Vertebrate Origins (protochordate study image;
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a) pharyngeal gill slits
b) endostyle
c) notochord
d) dorsal hollow nerve cord
e) post-anal tail
Cl. ASCIDIACEA (sea squirts):
- sessile, solitary or colonial, filter feeders
- adults bear little resemblance to other chordates:
- sessile & covered by a complex tunic
- have a pharynx with many gill slits
- lack a notochord and nerve cord
- the 'tadpole' larva possesses the distinctive chordate characters
- an endostyle secretes an iodinated mucous net that functions as a fine seive; cilia propel it across the pharynx and eventually to the mouth
Cl. LARVACEA (appendicularians):
- pelagic; make amazing mucous houses to suspension feed; make several houses per day (4 - 16); can filter particles < 0.1 micron!
- the likely sister group of all remaining urochordates
Copyright (c) 2004 by A. Richard Palmer. All rights reserved.
(revised March 25, 2004)