As a long-time student of invertebrate zoology I have for most of my life appreciated the immense variety and ingenuity of animal body plans. And one of the things I've always found the most intriguing is the pentaradial symmetry of echinoderms. I remember thinking, the first time I encountered a live echinoderm (probably a star at the beach, when I was in elementary school), "Wow. Five arms. That's weird." And now, all these years later, knowing a bit more than I did then, I still find it weird.
Pentaradial symmetry doesn't occur in any animal group except the echinoderms, and even they begin life as bilateral larvae. Remember these guys?
There isn't a more perfect example of bilateral symmetry out there. Although, even at this stage there are developments within the body that are beginning to interrupt the bilateral-ness of the animal. This is a picture of the animal lying on its dorsal surface, so you are looking down on its ventral surface. See how, to the (animal's) left of the stomach there is a darkish squiggle running mostly horizontally between the stomach and the skeletal rod of that arm, that you don't see on the right side? That squiggle indicates where the juvenile rudiment, which contains the first five tube feet of the water vascular system, will form.
As we've seen already, the rudiment grows to the point that it occupies most of the internal space of the pluteus larva. When the larva settles it lands on its left side, where the tube feet erupt during metamorphosis. The end result is (hopefully!) a little urchin walking around on tube feet that it didn't have the day before. Well, I guess technically it had them, but there weren't useful yet. And the body symmetry will have changed from the bilateral larval form to the pentaradial juvenile.
When looking at a live sea urchin it can be difficult making sense of all the stuff that's going on. A sea urchin is a very active animal, with spines and tube feet waving all over the place. It looks like total chaos at first, but examination of a naked sea urchin test (the endoskeleton made up of interlocking calcareous ossicles) lends a lot of insight into the body plan of this animal.
Here's a cleaned intact urchin test:
Now the pentaradial symmetry of this body plan becomes apparent. You can see that there are five regions of doubled rows of plates that have little holes in them. The holes are where the tube feet protrude to the outside, and the plates that bear them represent the animal's ambulacrum, or ambulacral region. The structures of the water vascular system run up along the inside surface of the test in the five ambulacra. The ambulacral regions are separated from each other by five intermabulacral regions, which do not have holes for tube feet because there are no tube feet here. The bumps on the test are called tubercles, and are where the spines attach. The tubercles fit into the base of the spines like a ball-and-socket joint, similar to our shoulder, that allows the spines to rotate 360˚. You can see this for yourself the next time you have a live urchin available: touch one of the spines and observe how the animal reacts.
There is interesting stuff going on at the apex of the urchin, too. The five large-ish holes, one at the point of each interambulacral area, are the gonopores. When I shoot up urchins to make them spawn, the gametes are released from these holes. The arrangement of the gonopores in the interambulacral regions makes sense, once you remember that on the inside of the test the ambulacral areas are where the water vascular system structures (including tube feet) are located. The only space available for the gonads is in the interambulacral areas. I know, it's confusing. And people think invertebrates are simple. Ha!
That may be enough to digest about urchin symmetry for now. I'll have more on this soon, including the implications of pentaradial symmetry. Stay tuned!