I think the hydra lacks the specialized cells for touch and chemical detection. On the other hand earthworms have touch, light, vibration and chemical receptors along the entire body surface. Both of them are invertebrates (those animals without a backbone), they include insects, worms, jelly fish, spiders among other animals.
There were two approaches for the planetary paths. The idea about the planets revolving around the Earth, the planetary paths were plotted and were erratic. It was not relevant to the laws of motion and gravity. That fact was neglected by the believers in an Earth-centered Solar System.
Sun-centered Solar System was theorized by Copernicus. The paths assumed to be relatively circular (or elliptical).
The hydroponics root system to grow is that they use the same method as the soil root but the root volume for hydroponics is highly restricted and they don't need to use too much energy to grow.
Answer:
Plants absorb nitrogen from: the soil ( second choice)
