This is the green sea urchin, Psammechinus miliaris, which is found in shallow water along rocky coasts around Britain and, in particular, in some of the rock pools along the Ormsaigbeg shore, where we went wandering this morning. This picture shows a typical specimen, with its spines almost olive green near its shell, changing outwards to a pale pinky-purple. Look closely and you can see the pale tube feet between its spines which enable it to move. These are operated by the same hydraulic system used in modern machinery - except the urchins invented it hundreds of millions of years before humans arrived on this planet.
These urchins aren't too easy to find, firstly because they have a habit of camouflaging themselves using anything loose that comes to hand - or, rather, tube foot. Some of them keep their camouflage simple, so a sheet of seaweed is fine, although....
....the less bright ones don't seem to realise that the object does need to be fairly large to be of any use.
Others, perhaps the vainer ones, have become quite arty about what they use, this one obviously realising that the inside of limpet shells are pleasingly pearly, while....
....there are always going to be those who take things to extremes, covering themselves in bling: we think there may be as many as four urchins buried under this pile of makeup.
The other reason why they're not that common is that they seem only to like pools which are shallow, scoured by the high tide, and lined with the pinky-white calcarious material - seen in this picture - which is formed by the appropriately named alga Lithophyllum incrustans. Because these pools, being shallow, warm up quickly, they may be high in the calcium carbonate which the sea urchins use to build their shells and spines.
This is a typical militaris-friendly pool. We found nine in it this morning, but there have been times, particularly in winter, when we've struggled to find any.