|A sea squirt (Ciona intestinalis) - a member of Tunicata |
Wikimedia Commons uploaded by perezoso (GFDL)
Phylum Chordata comprises three subphyla: Vertebrata (Craniata), Tunicata (Urochordata)and Cephalochordata. Tunicates include sea squirts such as Ciona (pictured) above. A familiar cephalochordate is the lancelet Branchiostoma lanceolatum better known as Amphioxus (shown below).
|Amphioxus or Branchiostoma lanceolatum |
(c) Virginia Gewin here (CC-BY-SA 3.0)
Chordates belong in the Superphylum Deuterostomia (brilliantly reviewed by Lowe et al. here) along with Phylum Hemichordata and Phylum Echinodermata. Echinoderms are richly represented in the fossil record and the five extant classes include sea urchins, sea cucumbers and starfish. Hemichordates include the acorn worms for which two genomes just became available (here and here).
|Acorn Worms (Hemichordata: Enteropneusta) |
from Spengel 1883 (public domain)
Relationships between deuterostome phyla were largely worked out through their embryology, an example being the erection of Chordata by Haeckel. Understanding the genes involved in developmental processes remains a focus in working out our evolutionary history (see the review by Lowe et al. mentioned above).