I have concentrations of one metal in two organs of a snail – kidney and foot (n=10). What statistical test should I use to check if the difference is significant between them?

Are the data paired or independent? If you had 10 snails, and had paired kidney and foot data for each snail, use a paired t-test. If the data are independent use a 2-sample t-test. If the data are not normal, there are non-parametric equivalents to these.

This assumes that you meant differences between organs. If you meant differences between snails, that is a different question.

In the first post there seems to be four factors (or five) and a mixed model because of within individual (foot kidney) effects and between individual effects.

I guess that the amount of metal will have many values close to zero and a few larger, so that it is a skewed distribution. Taking logs could possibly solve that.