In J.D. Salinger's A Perfect Day For Bananafish, Seymour, the main character, talks about a bananafish. He tells a little girl about the bananafish. The little girl says she sees the fish.
What is the bananafish and what does it represent?
The bananafish represents Seymour, and all the other returning soldiers.
"Well, they swim into a hole where there's a lot of bananas. They're very ordinary-looking fish when they swim in. But once they get in, they behave like pigs. Why, I've known some bananafish to swim into a banana hole and eat as many as seventy-eight bananas." He edged the float and its passenger a foot closer to the horizon. "Naturally, after that they're so fat they can't get out of the hole again. Can't fit through the door."
Sybil said, "What happens to them?"
"What happens to who?"
"Oh, you mean after they eat so many bananas they can't get out of the banana hole?"
"Yes," said Sybil.
"Well, I hate to tell you, Sybil. They die."
"Why?" asked Sybil.
"Well, they get banana fever. It's a terrible disease."
This represents Seymour pretty accurately.
He and other soldiers came into war as ordinary men, like the bananafishes were ordinary when they went into the hole.
In the hole (war), they became full of horror and/or pleasure at fighting, like the bananafishes eating the bananas and becoming full of.. well.. bananas.
They became trapped inside the hole (war) and diseased. In the case of the soldiers, in more ways than one. They became full of literal diseases, because the trenches are a nasty place, but they also became mentally ill.