Scallop Shell.

Our writing group threw up a surprise for me today having agreed, in my absence at the previous meeting, that they would produce a lucky dip bag and that we should each spend ten minutes writing something inspired by the item which we drew out. My inspiration was a child’s puzzle, but I was more attracted to this scallop shell and its incrustation.
 
In particular, the worm castes are created by Serpulidae, a family of sessile, tube-building annelid worms in the class Polychaeta. To paraphrase the words of my good friend Google, these worms extract calcium from the sea-water and use it to create a protective shell.
 
It is here that I digress a little. In my youth, while studying with the Open University, I trained a goldfish as part of an experiment to prove that it possessed colour vision. A feature of the experimental procedure was that it killed the poor fish before the conclusion was reached. Being a good student, I re-designed the method so that the fish survived long enough to become the family pet. Somewhere in the proceedings, I found a caddisfly larva and popped it into the aquarium; its case was made out of fragments of stems of vegetation gathered from its natural environment. Within the confines of its new home, it presumably discovered that small pieces of gravel had much better structural properties and so discarded its old home and created a much more robust structure out of the newly available material. It survived the predations of the fish happily for a few years before successfully transmogrifying into an adult. I mention this solely because I was struck by the very slight similarity between the caddis and the Serpulida in the way that they construct their homes.