Chordates


Reminder: Clicking on the picture will take you to the site where I originally found it.

I hope you feel at home here among the chordates. Really, you should. You belong in this phylum, along with some interesting critters. Welcome home! You are in good company. Take a look at some of the other members of the chordates.

This is quite an elite club - the chordates. To belong, you have to have the four following characteristics:
  1. You must have or have had pharyngeal slits. These are a series of openings that connect the inside of the throat to the outside of the neck. Woah! Don't you have neck slits? You used to. As embryos, all of us had these slits. Among some animals, like the fish, these will become gills. In birds, reptiles, mammals and some amphibians, these close up. Even though you have no slits now, you're still in the club.
  2. To be admitted, you must have a dorsal nerve cord. This bunch of nerves down the back is known as your spinal cord (hence the name chordates). No spineless jellyfish are admitted here! You've gotta have nerves, and lots of them, to be a chordate.
  3. Third requirement, you had to have a notochord at some point. This is a cartilagenous rod running underneath, and supporting, the nerve cord. What... you can't find your cartillagenous rod? You had one, but your vertebrae have taken over the job. All that remains of your notochord is the cartillage between the vertebrae. These are also known as "discs" in the spine.
  4. Finally, you must have a post-anal tail. I don't mean to get personal, but have you checked to see if you've got a tail? You have one, you know. Some chordates tend to hide their tails inside the main part of their bodies. The human tailbone is just that - a tail bone.
Another point of order you might find interesting. All chordates (and oddly enough the echinoderms too) are deuterosomes. This means that the anal end of the digestive tract formed before the mouth end. At any rate, it's nice to know the hows and whys of your own phylum.

Before we get to the nuts and bolts of the chordates, you should know that almost all of us have backbones. You do, I do, even sharks do, kind-of. We belong to the sub-Phylum vertebrata, and it's a nice place to be. There are only two major groups who have spinal cords, but no backbones. They are the Cephalochordata or lancelets, and the tunicates, or Urochordata.
Discover more about the:
Cephalochordata

There aren't a whole lot of cephalochords, but they have a lot of names. Most belong to the class Brachiostomatidae, which was formerly known as Amphioxus. If you want to use the common name for these little critters, just call them lanclets.

These animals look like fish, a little. They are worm-like and eel like as well. They spend most of their lives buried in the sand.

Like most critters that spend their lives buried in sand, they don't have much need for a brain. They have enough, I guess. They do have a strong notochord which helps them swim. Most of their time is spent waving their gills about and gathering food. They have no heart, but survive just the same. It's not much of a life, but you don't need much more than that to be a lancelet.

It was widely believed that amphioxus (as they were then called) were a primitive ancestor of all chordates, but this is in doubt. They have too many oddities (by comparison to us) to be ancestral. Instead, most scientists think that the lancelets diverged away from the rest of the chordates early in their history.

Urochordata

Ya gotta love these guys, if for no other reason than their common name. Sea squirts. Of course, scientists need to feel they're talking about something scientific when discussing Urochordata, so they call them tunicates.

This is an exceptionally primitive chordate. Actually, they're only chordates part of their lives. When they are young larvae, they have a round body and a tail - roughly resembling tadpoles. They have the customary notochord and nerve chord.

Something weird happens when the tunicates become adults though. Tail? Gone. Notochord? Gone. Even the nerve chord (which functions like a brain) - gone. The adults give up their chordate traits to settle down. This has led to a humorous (or at least sarcastic) commentary on how this compares to humans.

The adult tunicate (or big squirt) still keeps its gills. It uses them to feed from the water. The adult is pretty much a big bag which lets water in one siphon and out another. In spite of this, it still has a cute name.

Vertebrata

All the other critters - you, me, your neighbor's pets - are vertebrata. As these are the most familiar of animals, I've placed each class on its own page. Here they are:


All the way back to the animal page,
see sponges,
near the cnidarians,
meet the mollusks,
advance to the arthropods,
enquire about the echinoderms,
wade through the worms, or
over to the overview.