The five main characteristics are foot region, hard exoskeleton, bilateral symmetry, left and right limbs and a segmented body.
The surface area to volume ratio of the cell decreases.
This decrease is due to the fact that the volume of the cell increases at a greater rate than the surface surrounding the cell, the cell membrane, increases. This change is also visible when we are blowing up a balloon.
When considering the cell division, the ratio remains roughly the same. It is advantageous for the cell to preserve its surface area to volume ratio, since a higher ratio means that substances are able to diffuse into and out of the cell at a fairly higher rate.
I believe the thyroid controls most, if not all of those things. (Again, I'm not entirely sure.)
In order for gas to leave a planet it needs to be hot enough and to overcome the planets gravity. The outer planets are colder and are very big compared to earth.
