From the outside. It’s quite the shock, cut in half. From Scientific American:
Their blood is clear and, strangely, can accumulte extremely high qualities of a mysterious and rare element called vanadium. The concentration of vanadium in the blood of P. chilensis and other tunicates can be up to 10 million times that of the surrounding seawater. Just why and how these creatures are able to accumulate vanadium in such huge quantities remains unknown.
Its reproductive habits are unusual too.
It just happens to have begun to look and act like this slowly over time, in order to avoid predators – in a thousand slight modifications one in each generation, via natural selection acting on random mutation, right? Right?
Element Vanadium:
Pure vanadium is a greyish silvery metal, and is soft and ductile. It has good corrosion resistance to alkalis, sulphuric acid, hydrochloric acid, and salt waters. The metal oxidizes readily above 660̊C to form V2O5. Industrially, most vanadium produced is used as an additive to improve steels.
Despite the huge concentrations of vanadium, local Chileans eat this animal with salad and rice.
Note: The topic attracts its fair share of nonsense and then some. Don’t watch if it bothers you more than seeing shellfish opened.