Phylum Chordata

The phylum Chordata includes thousands of species found worldwide, but this guide will only cover a few classes. First, all chordates share the following four primary characteristics.

  1. A notochord (“back” – “string”), at least in the embryo stage, that serves as structural support.
  2. A dorsal hollow nerve cord, which serves to connect the brain and nerves with the rest of the body.
  3. Pharyngeal slits, essentially openings to the throat. In fish, the slits are their gills, and in humans the slits have been modified as our ears.
  4. A post-anal tail, or an extension of the body behind the anus. In humans, this gets absorbed before birth.

Since these phylum characteristics cover a wide variety of organisms, it is interesting to note that although two species may look very different, they can still be related. For instance, sea squirts first develop with a dorsal hollow nerve cord, meaning they are more related to humans than a sea slug!

As you read through the chordates in this section you will notice that they are grouped by classes, all of which are found in the subphylum Vertebrata. The following five classes are explored in the guide.

  • Osteichthyes (bony fishes)
  • Chondrichthyes (cartilaginous fishes)
  • Mammalia (mammals)
  • Reptilia (reptiles)
  • Aves (birds)
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