Notochord and Pharyngeal gill slits – Characteristic features of Chordates
- The dorsal, hollow nerve cord is basically a sheet of ectoderm rolled into a tube. A chordate's nerve cord develops into the brain and spinal cord in adults.
- The larval stage clearly exhibits chordate characteristics of tunicates.
There are three subphyla within Phylum Chordata: Vertebrata, Urochordata, and Cephalochordata. All members of the subphyla Urochordata and Cephalochordata are invertebrates (animals without backbones), as were all animals discussed previously (from porifera to echinodermata). All members of the subphylum Vertebrata are vertebrates (animals with backbones).
Subphylum Urochordata includes the tunicates ("sea squirts"), which are marine animals that are typically sessile as adults. The lightbulb tunicates pictured here on the right are adhering to coral. The adult tunicate, illustrated below , does not appear to possess any of the standard chordate characteristics.
However, the larval stage (rightmost figure below) clearly exhibits chordate characteristics; note the notochord, the hollow nerve cord, the pharyngeal gill slits, and the postanal tail.
In adults the gill slits are modified for filter feeding. Molecular evidence supports the idea that the urochordates are the group most closely related to the vertebrates.
Subphylum Cephalochordata includes the cephalochordates (lancelets), which are small marine animals that burrow, tail first, into the sand. In this position they are able to draw food particles into their mouths via their waving tentacles.