Amphioxus -- more than just a worm

We started the class by going through the homework.  Students did really well on the quiz about grasshopper and crayfish, and we had a lot of fun with the assignment, which was to come up with a quiz question for each of the four invertebrates we studied recently (earthworm, sea star, crayfish, and grasshopper).  The main point I made about the homework was that the key to doing it well is to read the instructions.  Seems simple but lots of kids and adults alike don't do it and then wonder why they don't get better grades.

Next, we did our quizshow.  The teams this week were the CrayWorms and the Sea Hoppers, because of the four animals we were studying.  Each student answered a question about each of the four animals, though we had some glitches with the software.  Classroom Quizshow is great software made by a guy named Stephen McNutt.  Here's his page where you could look at it and consider it you might want to play it in a teaching role or maybe even at a party.  It's really fun and flexible.  We voted this time on whether to keep using the Tic Tac Toe game or try a different one, and we ended up sticking with Tic Tac Toe.  But twice we ended up with Cat's games (neither team could win), and the software didn't reset the gameboard.  It was a hassle.  We worked around it, but just now I emailed Mr. McNutt to see if he might be able to fix that problem.

On to the dissection.  This week we studied Amphioxus, which is the first chordate we'll study.  It is actually considered a hemi-chordate, because it doesn't have vertebrae.  It has a spinal cord surrounded by a muscularized rod, and this structure is called the notochord.  One thing that is different from invertebrates that have a nerve cord and a chordates, which have a spinal cord or notochord, is that the nerve cord is dorsal.  It's on the back of the animal, like our spinal cord is on our back.  One thing I didn't mention in class is that it lives mostly in the sand, as you can see in the two pictures in this blog post.

So while amphioxus looks like just a worm, it's really different from the earthworms we looked at two weeks ago.  Some of its distinguishing features are pharyngeal gill slits, which strain food particles out of the water, the notochord, and the post-anal tail.  We also identified the fins -- it had a dorsal fin (on its back), a ventral fin (on its tummy), and a caudal fin (near its tail).  The mouth has buccal cirri, which are little tiny tentacles near its mouth that filter the food when it is sucked into the animal.

Water goes through the gill slits and out the atriopore, which was a small hole partway down the animal's body.  Food gets trapped in the pharynx, and then cilia move the particles to the esophagus.  It gets digested, and the esophagus extends to a stomach, which then empties into an intestine, which is a straight tube that ends at the anus.  There are two sexes, but we wouldn't be able to distinguish them ourselves, so we didn't bother trying.

This species is important, because scientists think that this type of animal is the ancestor of all vertebrates, and is part of how spinal cords first developed.  In my research to find an interesting article recently published about Amphioxus, I saw that actually a lot of work is being done on this species right now.  In particular, scientists are studying a lot of different genes from this species, probably to better understand how different genes appear and are changed in vertebrate species such as us.