Thursday, 1 April 2010

Some light from Aristotle's Lantern

I was going to clear out the garage but unfortunately, 2 minutes in, I came upon a bucket.  In it was lots of flotsam I picked up on the Northumberland coast last year and promptly forgot about.  Which brings me to sea urchins. Apart from the incredible beauty of the structural form of the external skeleton (or 'test'), two things are worth pondering. Did Aristotle really study the urchin's mouthparts 2,340 years ago and how does little become large?

So, to Aristotle first.  The Common Sea Urchin grazes encrusting growths from surfaces. Its mouth is underneath its body (and its anus on the top funnily enough) and it has 5 hard protruding teeth.

Inside the body, revealed below in this washed up skeleton, is the incredibly delicate and complex structure of bone, tendon and muscle that operates the mouth. Commonly known as Aristotle's Lantern - it is indeed named after the great man himself.  An accomplished naturalist as well as a philosopher and all round clever dick, he described it in his book Historia Animalium. He wrote, "In reality the mouth-apparatus of the urchin looks like a horn lantern with the panes of horn left out."  2,340 years later we still refer to it as Aristotle's Lantern. I rather like that.  It's so much more elegant than the phrase I came across in a 1922 book on the seashore, which referred to this as the urchins 'apparatus of mastication'!

Which brings me next to the little and large question.

I found a good description of the way sea urchins grow in a book called 'A Student's Guide to the Seashore' ,written - in as fine an example of nominate determinism as you are likely to come across - by JD and S Fish. (I kid you not).  The test is made up of beautifully interlocking plates and growth takes place around the edges of each plate where new calcite material is laid down.

From a millimetre or so in diameter when it first transmogrifies into a recognisable urchin from its planktonic stage, to about 4 cm across is achieved in the first year alone.  Fully grown it is usually about 100 to 120 mm across. Apparently, if you take one of the plates and polish it up with fine sandpaper you can see annual growth bands and estimate the urchin's age. Must try that one day.
Meanwhile, the garage can wait. Its late and I'm off to bed.... but not before I have cleaned my masticatory apparatus.

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