Jawless Craniates: Class Myxini & Class Petromyzontida

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  • 0:02 Jawless Craniates
  • 0:50 Class Myxini - The Hagfishes
  • 2:29 Petromyzontida - The Lampreys
  • 3:41 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

The word 'fish' brings specific images to mind. But before these fish existed, there were the jawless craniates. In this lesson you will identify two classes that fall into this category: the hagfishes and the lampreys.

Jawless Crainiates

I don't give my jaw enough credit, but it really does a lot for me. It allows me to chew, talk, make funny faces, and so much more. I can't imagine my life without it! There are some animals that don't have this luxury though. A few of these are evolutionarily related to us because they are both chordates, a classification of animals that includes humans and many other vertebrates, and craniates, which are chordates with a head. But that's where the relationship ends.

This branch of the evolutionary tree contains some of the most primitive surviving craniates. Here, the word primitive doesn't mean simple; it means old or ancestral. This is why we don't look much like these distant relatives - they branched off of the chordate tree a very long time ago!

Class Myxini - the Hagfishes

One of these primitive, jawless craniates is a group called the hagfishes, which represent the class Myxini. There are about 40 species found on Earth today, and they really are old! They are craniates, but they aren't actually considered vertebrates because they don't have a backbone. Instead, they have a strong, flexible rod through their body called a notochord. As the body's main support, the notochord serves a similar function as the backbone, but it's not quite the same thing.

Simply put, hagfish are cool. They live on the seafloor, which is dark and cold. They are pretty much blind, but since there's so little light down there they don't really notice. Instead, they have excellent senses of smell and touch, which helps them scavenge the seafloor for dead or dying animals. Once they find one, they make a hole in the animal and head inside for dinner.

Hagfish also have a great parlor trick - they make slime! In fact, the word 'myxo' (from which Myxini is derived) means 'slime.' When they are threatened, hagfish expel an incredible amount of slime from glands along their sides - up to a gallon at one time, but combined with water it can expand up to several gallons! This makes it really hard to grab onto the hagfish, helping them escape predation and harm.

But how does a hagfish get going again if it is covered in slime? This is the best part of the trick: it ties its body into a knot, slides the knot forward, and peels off the slime. After this, it is free to go on its merry way again.

Petromyzontida - the Lampreys

Lampreys, which make up the class Petromyzontida, may look similar to hagfish, but there's a major distinction between the two - lampreys are vertebrates. In fact, lampreys are thought to be the oldest living lineage of vertebrates. Some lampreys are parasitic, but many are not. Some even migrate between freshwater and marine environments for different life stages.

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