A. epithelial cells form from the body's skin
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.
The number of chromosomes in a sex cell is half the number of chromosomes in a body cell. Think of it this way: 1 egg (women sex cell) combine with 1 sperm (male sex cell) to make 1 whole cell.
