I decided to draw two variations, terrastial and aquatic figuring out that the species adapted to living on land (what happened gradually and slowly) would have to be less amorpheous to maintain itself in air which has lesser density than water. These guys are big!
"They were normally shapeless entities composed of a viscous jelly which looked like an agglutination of bubbles, and each averaged about fifteen feet in diameter when a sphere. They had, however, a constantly shifting shape and volume - throwing out temporary developments or forming apparent organs of sight, hearing, and speech in imitation of their masters, either spontaneously or according to suggestion." (...) It was a terrible, indescribable thing vaster than any subway train—a shapeless congeries of protoplasmic bubbles, faintly self-luminous, and with myriads of temporary eyes forming and unforming as pustules of greenish light all over the tunnel-filling front that bore down upon us..."