Jayne has taught health/nutrition and education at the college level and has a master's degree in education.
Most of us are familiar with several of the animals that make up the phylum, or division, Chordata, meaning 'cord,' because it includes humans and many other vertebrates, such as cats and dogs, and they live in marine, freshwater, and land habitats all over the world. Chordates are animals who are vertebrates (having a backbone) or closely related invertebrates (lacking a backbone). Vertebrates and invertebrates, you say? What makes them related? Good question. Let's find out.
If we were to compare a sea squirt and a polar bear, we wouldn't find much in common, would we? The sea squirt is mostly just a mass of transparent jelly with no brain and no sensory organs, and a polar bear has four legs, fur, and excellent senses of smell, eyesight, and hearing. How could they possibly be related?
Well, all chordates have the following features at some point in their lives. Generally, that point is during the embryonic stage (developmental). Many of these features disappear in the adult stage, as in our example of the sea squirt and the polar bear.
- Gill slits - these appear in the throat and show the aquatic ancestry of the chordates
- Dorsal nerve cord - this cord contains nerve fibers and develops into the spinal cord and brain
- Notochord - consisting of cartilage, this is the ancient version of the vertebra or backbone
- Post-anal tail
Let's find out what groups make up the Chordata phylum.
The Chordata share many developmental features, which is how they are grouped. As embryos, they are quite similar, however, these animals are very different from each other as adults. Because of this, the Chordata are divided into three sub-phyla (sub-divisions). These are the Tunicate, the Cephalochordata, and the Craniata.
The Tunicata, meaning 'tunic,' have no brain. The larvae have both a notochord and a nerve cord, which both disappear in the adults. The larvae look similar to tadpoles. As they mature, they lose the ability to swim and anchor themselves to a rock or similar hard surface. The adult looks like a bag enclosed in a slightly harder covering (the tunic) with two siphons for water to enter and exit. As the water circulates through this bag body, food is filtered out, and the water is squirted back into the sea. These are referred to as sea squirts. The free-floating ones are called salps.
The Cephalochordata (cephalo meaning 'head' and chordata meaning 'cord') also have no brain, although they do have a notochord and a nerve cord and very simple circulatory systems. There are about 25 species of Cephalochordata. They are small and eel-like. They are often called lancelets because of the shape of their bodies. They prefer coastal waters of the tropical and temperate areas of the world. They burrow into the sand in shallow waters where there is a current bringing food they can grab with their tentacles.
The last sub-phyla of the Chordata are the Craniata, meaning 'cranium.' This group is composed of vertebrates and hagfish and is the only group within the Chordata that have skulls. All craniates have a dorsal nerve cord surrounded by either cartilage (notochord) or bony vertebrae (backbone). Hagfish have partial skulls but no vertebrae, so they are not vertebrates. True vertebrates have a brain, a skull, plus a backbone.
Hagfish are almost blind but have developed organs for touch and smell. They have no larval stage. They have a partial skull but no vertebrae because their skeleton is made of cartilage, and they have no jaws. Due to the lack of jaws, they were originally classified with the lamprey in the Agnatha group.
Let's find out more about how classification works.
Classification of the Chordata has been difficult because the fossil record is lacking in samples. This is because the Tunicata and the Cephalochordata have soft bodies with no hard parts to be fossilized. Remember that bag-like body? The current theory of classification is that Chordata contains all the descendants (offspring) of a single common ancestor who was a chordate, although we don't yet know who that ancestor was. So far, it appears that the nearest relative of the craniates are the Cephalochordates. One of the few cephalochordate fossils from the Cambrian period was found in China.
Chordates are animals who are vertebrates, or closely related invertebrates, that have the same features at some point in their lives, such as gill slits, a dorsal nerve cord, a notochord, and a post-anal tail. The Chordata are divided into three sub-phyla: the Tunicata (sea squirts), the Cephalochordata (lancelets), and the Craniata (vertebrates and hagfish). Classification of the Chordata is difficult due to a lack of fossils because the Tunicata and Cephalochordata have no hard body parts to fossilize.
After you are done reviewing this lesson, you should be able to:
- Characterize the members of the phylum Chordata
- List the embryonic features Chordates share
- Name and describe the three sub-phyla of Chordata
- Explain why it is difficult to classify the Chordata
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