Sometimes I think we don’t fully appreciate just how alien arthropods are compared with mammals and other vertebrates, simply because (a) they are all over the place and we are used to them, and (b) they are mostly so small that we don’t clearly see the extent to which they do things very differently from us. Relative to us, in fact, they could be considered to be constructed backwards, upside-down, and inside-out. Here, let me explain what I mean by this [1].

1. Backwards? (digestive tract)
The last common ancestor of vertebrates and arthropods appears to be the (so far hypothetical) “ur-bilateran”, a bilaterally symmetrical, probably macroscopic organism with a photosensitive eye-spot, a bare minimum nervous system, and either no digestive tract at all, or a digestive cavity with a single opening that served both to bring food in, and to move wastes out. Here, let me make a quick sketch:

(also, see Xenoturbella)

There are a lot of animals, like Coelenterates that still only have a single opening like that. This works OK, but they have to eat in batches – engulf something, digest it, then spit out the remains. But, this limits the speed and efficiency of eating. As has been found in chemical engineering processes, you can generally get a much higher throughput from a continuous process than from a batch process. A continuous process allows you to to keep putting in raw materials at one end, and extract the products at the other end. This would mean that a continuous-processing animal could process more food in a given amount of time with any given amount of digestive tract volume. And, of course, making it continuous means that there has to be a direction of flow, from an inlet (mouth) to an outlet (anus).

The first split between vertebrate and arthropod ancestry, was essentially a Backwards/Forwards split. Both arthropods and vertebrates are “two-holers”, but they differ in the order in which they generate their holes.

To get from one hole to two holes, embryos start as a ball of cells, which develops a depression on one side that will make the first hole. That depression then propagates through the forming embryo, until it comes out the other side, forming the second hole. The question now is this: is the initial depression going to be the mouth, or is it going to be the anus?

If the first hole (the blastopore) becomes the mouth, then the organism is a protostome, the group that includes arthropods. On the other hand, if the first hole becomes the anus, then it is a deuterostome, and vertebrates are a part of this group.

So, this means that we could consider arthropods to be oriented backwards relative to us. Or, alternatively, since the protostomes are much more numerous than the deuterostomes, that would make us the backwards ones.

2. Upside Down? (nervous system)

So now that we have a digestive tract laid out, the next question is the nervous system. The original ur-bilateran probably only had a simple nerve-net nervous system, with no particular brain, and so after the deuterostome/protostome split, our respective ancestors laid things out pretty much independently from each other.

The basic vertebrate nervous system is an organized mass of nervous tissue that makes up a brain, which receives sensory inputs and sends motor impulses through the rest of the body. The brain is located in the head region, to put it close to the sensory organs, which in turn are located close to the mouth to make it faster and easier to locate and ingest food. To control the rest of the body, vertebrates run a single nerve trunk along the backbone, with branches going off to the rest of the body as needed.

Arthropods also have a nervous system, but they have different issues. They still have a brain next to the mouth, but they also have a segmented body with legs (or other appendages) coming out of the bottom of each segment. These legs need to receive nerve impulses, and so it works best if the nerves run along the belly of the creature, instead of along the back. Arthropods therefore have a dual nerve cord, with a ganglion in each body segment to control the legs.

So, if we use the main nerve cord to define the orientation of the body, we see that vertebrates have theirs in the back while arthropods have theirs in the belly, so one of us has to be upside-down relative to the other.

3. Inside Out? (exoskeleton vs. endoskeleton)

Arthropods and vertebrates have taken completely opposite approaches to body stiffening and providing a solid support for muscles to attach to. Vertebrates obviously have our bones inside, with the rest of the body attached to this internal skeleton (endoskeleton)[2]. Arthropods, on the other hand, basically have stiffened, armored skin that is hollow, and the rest of their body is stuffed inside of it. So while a vertebrate skeleton is hidden within the surrounding flesh, with an arthropod all that we see is the skeleton, and everything else is hidden away. So, once more, if we compare arthropods and vertebrates to each other, one of us is inside out.

++++++++++++++++++++ And on a semi-related note +++++++++++++

Speaking of exoskeletons and who does and does not have a backbone, there is something that I have been seeing lately in Halloween decorations that I have to get off my chest. Many places are selling these “spider skeletons”. This particular one is from WalMart, but do a search for “spider skeleton decoration” and you’ll find hundreds of variations on it, as well as other traditionally “creepy” arthropods like scorpions.

There is one huge problem with this – it is Absolutely, Completely, 100% WRONG! The wrongness positively drips off of it! Remember what I wrote above, about how with an arthropod, all you see is the skeleton? SPIDERS DON’T HAVE AN INTERNAL SKELETON LIKE THIS. AT ALL! NOT EVEN SLIGHTLY! And certainly not a rib cage. If you want to display a spider skeleton, just display the spider! And then to top it off, see how there appears to be a backbone running along the back of the abdomen, supporting the ribs? SPIDERS DON’T HAVE ANYTHING LIKE A BACKBONE! If they did, they would be vertebrates. That’s what vertebrate means.

Yes, yes, I know. It’s just a decoration. But still. Didn’t these people pay any attention at all in biology class?

Oh well, I suppose it could be worse, they could be selling “skeleton” decorations for animals that don’t have skeletons of any description, and in fact are well-known for being completely boneless, like an octopu . . . NOOOOOOOOOOOOOO!

that does it. I give up.

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[1] I decided to draw the figures myself here, for two reasons: First, this way I don’t have to worry about copyright violations. And second, this way I can show just the exact thing I want to explain, without a lot of distracting artistic touches. I’m not under any illusions that I am any kind of artist, but I think I can do OK line diagrams at least.

[2] The vertebrate internal skeleton is actually pretty unusual when you look at it. No other animal lineage that I am aware of has independently evolved an internal jointed skeleton that even vaguely resembles ours. Arthropods have jointed exoskeletons, which I have already noted is pretty much the inverse of ours. Worms either have nothing, or an exoskeleton broadly similar to arthropods. Jellies obviously have no bones at all. Molluscs mostly either have no skeleton at all (like slugs and octopuses) or an external shell that has little or nothing in the way of joints (like snails and clams). Cuttlefish have an internal “bone”, but it is just an unjointed stiffener. Echinoderms (sea stars, sea urchins, sea cucumbers, and sand dollars) have an unjointed skeleton embedded in their skin. Sponges have a “skeleton”, but it is just internal stiffening with no joints. It really looks like the development of an internal jointed skeleton was a one-time evolutionary development. If the original ancestral vertebrate had gone extinct without leaving descendants, it is pretty likely that nothing even vaguely similar to vertebrates would have evolved.