If you have 10-15 minutes to spare, click through to this beautiful piece by P. Z. Myers entitled “Pycnogonid tagmosis and echoes of the Cambrian”. Sounds terribly esoteric and dry, doesn’t it? But it’s not: it’s a delightful example of how science is done. It begins:
The evolutionary foundation for the organization of many animal body plans is segmental—we are made of rings of similar stuff, repeated over and over again along our body length. That’s sufficient to make a creature like a tapeworm or a leech (well, almost—leeches have sophisticated specializations), but there are further steps involved in making a fly or a spider or a human. There is an arrangement of positional information along the length of an animal, so one segment can recognize whether it is near the head or the tail, and the acquisition of new patterns of gene expression based on that positional information that cause the development of specialized structures in different segments. That process of specializing segments is called tagmosis. It’s how a fly forms mouthparts in head segments, legs and wings in thoracic segments, and no limbs at all in abdominal segments.
And then he dives into an account of the pycnogonids or sea spiders, and how their heads are structured, and how we know (hint: it involves enervation), and how this structure evolved. Even if (like me) you’re not familiar with all of the terms or concepts, that’s OK: Myers’ writing is accessible, satisfying, surprising, funny, and really elegant. If you enjoyed Dawkins’ The Ancestors’ Tale, you’ll love this.