The energy flows from the plant to the snail. The snail eats the plaint as well as consume oxygen from it too. From there the snail produces body heat and even carbon dioxide. Then the process repeats. Photosynthesis in the plant requires water, sunlight, and carbon dioxide; the snail eats the plant/ intakes oxygen etc. Hope that helps. :)
if you need clarification please comment I'll help you out.
