I had been following the development of my cacti which were pretty sad when we bought them but have been coming along well with lots of branches. Reading Tom Stuart-Smith's guide to the Secret Garden I was surprised to learn that they aren't cacti at all. In fact they are a type of Euphorbia, Euphorbia ingens. All euphorbia have an irritant sap so I won't have them in my garden in Wales. The latex of E ingens is particularly toxic and can cause not just skin irritation but Blindness and severe illness. I certainly wouldn't have bought them had I realised but they seem a bit dangerous to get rid of.
I was also surprised to learn that Agaves and Aloes aren't closely related at all their similarities being a result of paralell evolution. I think I have both but can't tell which is which.