Darwin is a fish with arms and legs. His goldfish-shaped head takes up his entire body, with his fins and legs hanging down from it. He has retained his goldfish tail and fins, the latter of which he now uses as arms and hand. His legs are much longer than his body, ending with plain feet matching the color and texture of the rest of his body. Usually, though, he covers them with green and white sneaker. Darwin is also shown to have a bubbly butt on the back of his head, as revealed in the orginal trailer. A common misconception in Season 1 was that he also wore a black shirt as well as pants, but that was actually just a shadow cast by his head.
Darwin apparently has thick coatings of blubber, making him resistant to pain, as he is shown unharmed in "The Responsible" after a painful face-first slap on a concrete sidewalk from way up in the air. The blubber's depth is about the length of Gumball's arm, as shown in "The DVD", where Gumball had to sink his whole arm into Darwin's head just to reach his cheekbone. Darwin also thinks he has perfect skin as shown in "The DVD". Darwin is shown naked in "The Meddler" without his shoes, but his feet are censored for some strange reason, possibly as a joke due to the fact that he only wears shoes. His feet were also shown in "The Goons", "The Club", "The Skull", and "The Dream", but uncensored, In "The Dream", he says that he considers his feet worthy of a censor because it is the only place he wears clothes.
In season 2, Darwin gets a redesign. His black shadow is more transparent, almost to the point where it's not noticeable. Also, his legs get slightly fatter, and he loses one eyelash on each eye (two on each eye instead of three). Like Gumball, his eyes are more round. His voise also started to sound slightly deeper, and whenever he screams, his voice squeaks, this is probably because of Kwesi Boakye, his voise actor, entering puperty.
In the Shadow of Dead Island
The Night He Came to Elmore
Animaniacs: Elmore's New Trouble Makers