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Chordates make up most of the critters you consider animals. This lesson will explain what nervous system characteristics this phylum shares and then will look at the sub-phyla within this group.

Chordata Phylum

What does an owl, a goldfish and a sea squirt all have in common? They’re all members of the Chordata Phylum (sometimes we’ll just refer to these critters as chordates, for short). All members of this group share certain characteristics.

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Before we delve into the nervous system of the chordates, or the portion of a chordate that transmits nerve impulses, thus allowing it to respond to the environment, let’s take a moment to check out the similarities within this phylum. Now, some of these characteristics are only present during the embryonic stage, but since they have them at some point in their life, it counts. Okay, here we go! And, throughout this lesson, you may start to notice a ‘cord’ theme…

and you’d be correct! Anyways, they all have (or had):

  • A notochord. Uh, a noto-what, you ask? It’s a rod made of a cartilage-type material that runs from the head to the tail of the critter. You might be thinking ‘I have a spinal column and not a notochord’, and you’d be right. However, during the embryonic stage of development, you had a notochord, which makes you a member of the chordata phylum. Just like a sea squirt! A notochord helps support the body, and in vertebrates (like you) it eventually becomes the spinal column.

    And here’s a fun fact, the name ‘Chordata’ comes from the fact that critters in this group have a cord running through their body. Get it CHORData, cord?

The sea squirt has a notochord during its larval stage so it is a member of the chordata phylum
sea squirt
  • Dorsal nerve cord is another feature the chordates share. This is a hollow nerve cord that is towards the back compared to the notochord. In animals that have a backbone (vertebrates) this dorsal nerve cord becomes the brain and spinal cord. In other groups, it forms a simple nervous system. If you keep taking biology, you’ll learn that the word ‘dorsal’ means towards the back (think: dorsal fin in fish as the fin on the back).

  • Okay, onto the third shared characteristic, or pharyngeal slits, which are openings near the mouth. And no, you don’t have these slits now, but you did when you were an embryo. Some aquatic members of the chordata phylum have these silts as adults.
  • Finally, chordates all have a post anal tail, which is just a fancy way of saying they all have tails. Again, you may be wondering why you fit into this group since you don’t have a tail. And again, you DID have a tail, but you were a teeny, tiny embryo and it was gone by the time you were born. Also, you DO have a tail bone, evidence of this tail.

Note the tail in the human embryo
embryo

We talked a little about the shared characteristics, but let’s get a little more specific and delve into the different nervous systems chordates possess. The chordata phylum is divided up into three sub-phyla, and each group has slightly different features, including different nervous systems. The three subphyla are as follows:

  • Urochordata (tunicates)
  • Cephalochordata (lancelets)
  • Vertebrata

The first two subphyla are pretty small with only 2,000 species combined. Vertebrates, however, have nearly 42,000 species. Let’s look at all of these subphyla now!

Nervous System of Urochordata

Let’s start with the Urochordata, which are sometimes called tunicates, and includes the really simple chordates, like the sea squirts.

These chordates have a body that is like a bag with a hole on each end, and water goes through the bag where tiny food floating in the water gets filtered. Most adults are sessile, meaning that they don’t really move around. These critters are considered chordates because they have chordate characteristics when they are in their larval stage, which they don’t have as adults. So, as adults, they don’t really seem to fit into the phylum chordata.

This group has a simple nervous system with ganglion, or a group of nerve cells, in-between the two holes (siphons). From the ganglion, nerves can grow to other areas of the body.

Members of Urochordata have a simple nervous system
seas

Nervous System of Cephalochordata

The members of the subphylum Cephalochordata sort of look like teeny-tiny fishes.

They’re filter feeders, like the members of Urochordata, and they can be found in the sand of marine environments.

The lancelet is in the subphylum Urochordata
lancelet

This group has a dorsal nerve cord that sits below the notochord and nerves branch out from there and go to the rest of the body. They don’t have a brain like more complex living things like us, but the notochord goes into the head region.

Nervous System of Vertebrata

So the Cephalochordates and the Urochordates are pretty simple, but the members of Verebrata are not. In vertebrate, or vertebrates, the notochord develops into a backbone (or vertebrae). Hmmm…

I guess that’s why they get the name vertebrates! This group of critters encompasses what you think of when you think of an animal — think dogs, cats, fish, birds and even you! The nerve cord has become a brain.There’s a central nervous system (CNS), which is the brain and spinal cord, and a peripheral nervous system (PNS), which consists of nerves outside the brain and spinal column. These two systems work together.

Sensory information, like sound, touch, sight, etc., is gathered and sent via the spinal cord to the brain where it is processed. The brain then tells the body what to do in response. Pretty fancy!

Lesson Summary

All members the Chordata Phylum share certain characteristics at some point during their development; they can include things as simple as a sea squirt to something as complex as a dog or even a human! Let’s take a moment to review these.

  • They all have a notochord, which is a cord that runs the length of the critter’s body.

    In vertebrates the notochord eventually becomes the spinal column.

  • They have a dorsal nerve cord, which is another cord, however, it is towards the back of the animal compared to the notochord. In vertebrates this becomes the brain and spinal column.

  • Next is the pharyngeal slits, which are openings near the mouth.
  • And lastly a post-anal tail, which just means a tail.

Remember, these occur at some point during development, so no, you don’t have some of these structures but you, as a human, still fit into the phyla.

There are three sub-phyla. The first, Urochordata, consists of sessile adults that have ganglions (or a group of nerve cells, in-between the two holes), however, they get clumped into the chordates because they fit the criteria when they are larval. The next, Cephalochordata, move around but they also have a really simple nervous system. Vertebrates have a more complicated nervous system consisting of complex central (the brain and spinal cord) nervous and peripheral (nerves outside the brain and spinal column) nervous systems.

So it’s hard to think you have much in common with a sea squirt, but, alas, you do!

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