Arrow Worms

For the creature of the month, I thought I would steer away from the usual large animals that I've been discussing (or at least relatively large). So, I discovered some very intriguing microscopic organisms that live in the ocean. Having gone through them, I decided the most interesting to talk about would be the arrow worm.

Arrow worms, also known as Chaetognaths, are "translucent dart-shaped animals..." Their bodies are divided into three sections: the head, the trunk, and the tail. On the head are anywhere from 4 to 14 grasping spines which flank a hollow area which contains the mouth. These spines are used as hooks to bring food into the mouth of the creature (which is probably why the spines are located around the mouth...). As you probably guessed, arrow worms are carnivorous. Unfortunately, they're not awesome enough to prey on larger creatures, so they just eat other planktonic animals.

Their mouths are comprised of one or two rows of tiny teeth. They have no respiratory system, nor do they have a circulatory system, although they do have a nervous system, which we'll come to later.

They generally have one or two pairs of fins on their trunk, except they're not made of the sort of stuff that most fish's fins are made of. Instead, the fins are made of the basement membrane extending from the epidermis. If you've taken human biology recently, you may remember that the basement membrane is the part of the skin that prevents things from getting into you through your skin, and the epidermis,is the outermost layer of the skin. Similarly, in arrow worms, the basement membranes that make the fins are thin sheets of fibers. And some arrow worms even have bioluminescent cells on their fins.

What makes some arrow worms even cooler is the fact that some arrow worms use the neurotoxin known as tetrodotoxin. Basically, it "blocks action potentials in nerves by binding to the pores  of the voltage-gated, fast sodium channels in nerve cell  membranes, essentially preventing any affected nerve cells from firing by blocking the channels used in the process." Pufferfish use a similar poison, and trigger fish carry a similar poison. There is no working antidote for humans, which means you probably don't want to be bitten by arrow worms...except, arrow worms can't bite you, since they're microscopic. So no worries.

They do have a simple nervous system, but it's fairly simple; they don't really have brains, but they do have two compound eyes. Once arrow worms get food into their mouths, it passes through an area where it is lubricated, and moved down an intestine, where most of the digestion occurs, and then the rest is disposed of. Other non-food materials are moved around by cilia inside their bodies.

Here's a picture I found. I made it extra large so you can see all its anatomy. The nice thing about microscopic organisms is that it's easy to identify body parts on them...