OK, the horsehoe crab has some fancy names associated with its external anatomy. Because it's related to spiders and scorpions, it often uses some of the terminology associated with them. So when we look at the front here — in a crab you would call this the cephalothorax — and in a horseshoe crab it's called the prosoma. And in the back here, the abdomen is called the opisthosoma. And then, of course, the telson. So those are the three main parts: the prosoma, the opisthosoma, and the telson. And if you look here, you see the indentations here, and often people ask about these indentations. What are they for? Well, since it's an invertebrate, the invertebrate has to have a place for the muscles to attach, so the muscles on the inside of the shell — these are actually little bumps on the inside for muscle attachment. That's what these little holes, so to speak, these little sutures here on the back are actually for muscle attachment in the animal. If you look here, you can see the large muscel here which attaches and makes the prosoma and the opisthosoma flexible.