Number in nouns and adjectives refers to...

 Numbers are usually adjectives, because the information they give is how many of the noun. They can be cardinal (like one, two, three), or ordinal (like first, second, third).

Explanation:

they need to keep water moving over their gills so they seem to have active periods and restful periods instead of undergoing deep  sleep.  they sleep swim

explanation:

viven is in the blank