How does Jerry’s perception of the tunnel change?

the dark shelf at the place where his feet had gone. He could see the hole. It was an irregular, dark gap; but he could not see deep into it. He let go of his anchor, clung with his hands to the edges of the holes, and tried to push himself in. He had got his head in, found his shoulders jammed, moved them in sidewise, and was inside as far as his waist. He could see nothing ahead. Something soft and clammy touched his mouth; he saw a dark frond moving against the grayish rock, and panic filled him. He thought of octopuses, of clinging weed. He pushed himself out backward and caught a glimpse, as he retreated, of a harmless tentacle of seaweed drifting in the mouth of the tunnel. But it was enough. He reached the sunlight, swam to the shore, and lay on the diving rock. He looked down into the blue well of water. He knew he must find his way through that cave, or hole, or tunnel, and out the other side.