Nematode Body Structure. The phrase tube-within-a-tube is a convenient way to think of nematode body structure, and also a term used to refer to a major trend in the evolution of triploblastic metazoa (Brusca and Brusca, 1990 Invertebrates).
Neutrals and Universal Colors. Let me explain in details... In general, neutrals include black, white, gray (mix black with white and you've got gray) and different shades of brown. Nowadays a color can be called a neutral based on how effectively it can blend in with other colors. Basically, they match with all colors and color combinations - and don't create much of an emotion.
The meaning of grey is often a calm, detached, quiet color that is lacking in energy. It is the middle product of black and white, thus it is very neutral and indecisive - a color which doesn't care to take sides. Because of its equal mixture of black and white, grey is a neutral, indecisive and detached color.