To calculate t (actually, the test statistic of an ANOVA is called F), you'll need to use an online calculator. This one: http://www.physics.csbsju.edu/stats/anova.html
is really easy to use. When it gives you a 'probability', that is your p-value. The p-value is what most people will be looking for, but typically you report both the t or the F as well as the p-value.
If these 'outside causes' affect your sample randomly, then yes, this is what the p-value tells you.
I was envisioning you plotting the value for each container as a dot. Thus, you would have five columns (representing your five treatments) with five dots each. You could technically make a boxplot with mean and error bars, but with a small sample size like five I would just stick with the dots. Does this make sense?
Let's say that the average changes in worm length for containers in treatment A were 5, 6, 7, 5, and 6 mm and the average changes for treatment B were 10, 9, 7, 8 and 9 mm. You would put 'mm' on your y-axis and have two columns, one labeled 'Treatment A' and one labeled 'Treatment B' on your x-axis. In the A column, you would have two dots at 5 and 6 mm and one at 7; in the B column you would have two dots at 9 mm and one each at 7, 8 and 10 mm.