Characteristics of Chordates

• The Hemichordata ("acorn worms") are mentioned here because they appear to be intermediates between echinoderms and chordates because of their gill-slitted pharynx and dorsal tubular nerve cord. They are in a separate phylum and are of phylogenetic interest.
• The phylum Chordata consists of a majority of species that are vertebrates (with a backbone) and a minority that are invertebrate chordates.
• All chordates, at some time in their lives, have four distinctive features:

The notochord is a long rod of stiffened tissue that supports the body; later it changes to bony units in vertebrates.

A dorsal, tubular nerve cord lies above the notochord and gut.

The muscular pharynx with gill slits is positioned at the entrance to the digestive tract.

A post-anal tail, or rudiment thereof, exists posterior of the anus.

• Chordate Classification consists of three subphyla: Urochordata, Cephalochordata, and Vertebrata.
• The vertebrates are divided into eight classes (one of which is extinct).
• Invertebrate Chordates consist of animals that lack a vertebral column and a cranium. They have the four chordate characteristics.
• Subphylum Urochordata includes the tunicates or sea squirts. They are marine organisms covered with a gelatinous outer coat or tunic.
• The larval stage resembles a tadpole and has a mid-ventral notochord in the tail with a dorsal nerve cord.
• Metamorphosis to the sessile (attached) adult results in a loss of the notochord and tail, a regression of the nerve cord, and an expansion of the pharynx for filter feeding. When metamorphosis occurs, the animal loses those features which adapt it to a swimming tadpole form.
• The sessile adult maintains a constant flow of water through the gill slits into the body to bring in food particles and oxygen and carry away wastes.
• Subphylum Cephalochordata includes the lancelets of which the most common is the genus Amphioxus. This group is considered to be most like the vertebrate ancestor.
• Lancelets are small, fishlike animals with tapered bodies.
• They lie buried in the sand filtering food from the stream of water passing through the pharynx.
• Muscles are arranged in a segmented pattern on both sides of the notochord; circulation is closed (but no red cells); respiration is directly across the body wall.
• Lancelets display all four of the vertebrate characteristics throughout their lives and do not possess metamorphosis.
• When Vertebrates evolved, the single, continuous notochord was replaced by a column of separate, hardened vertebrae, parts of which became modified near the head to form jaws.
• More complex sense organs and nervous systems began to arise in fishes.
• The fins of fishes were the starting point for the legs, arms, and wings seen among higher vertebrates.
• Gradually, there was less reliance on gills and more on lungs and the circulatory system (heart, blood vessels), which work in connection.
  

Below are internet links for the Hemichordata, and the two invertebrate subphyla, Urochordata and Cephalochordata. The Vertebrates are discussed by class and you should return to the previous page to continue examining them.

Hemichordata. University of Paisley, Scotland, site introducing the hemichordates. This unusual group seems to be closely related to the chordates but is placed in a separate phylum.

Introduction to the Urochordata. University of California, Museum of Paleontology site which presents a good introduction. Notice the incurrent and excurrent siphons on the pictured adults.

Urochordata. Diagram of the adult structure. Notice prominent pharyngeal gill slits, but reduced nerve cord and absent notochord and tail.

Subphylum Urochordata. University of Paisley, Scotland, site. Another basic introduction. Click on the links while reading the page to view diagrams and photos.

Urochordates. More detailed treatment of the subphylum from Bellarmine College. There are many links here to detailed diagrams and photos.

Subphylum Cephalochordata. Another University of Paisley site examining the cephalochordates. Check the link on amphioxus which shows a photo of the animal's unusual burrowing behavior.

Introduction to the Cephalochordata. University of California, Museum of Paleontology site which presents a good introduction.

Cephalochordates. Very good review of the cephalochordates from Bellarmine College. Click on all the links.