One is attractive, tall, and talented, while the other is short, awkward, and disproportioned (A).
The following passage describes the two oyster thieves from the Ghost:
"The boat we were discussing, the Ghost, lay directly beneath us. Two men composed its crew, one a squat and broad-shouldered fellow, with remarkably long and gorilla-like arms, the other, tall and well-proportioned, with clear blue eyes and a mat of straight black hair."
The Porpoise is described as well-built ("tall and well-proportioned"), whereas the Centipede is short and weirdly proportioned ("squat" and "with remarkably long and gorilla-like arms").
In the story, the Porpoise is also known to be a talented swimmer, hence his nickname, and the only one who manages to swim to shore after the rising tide.
