The water grew a darker blue as they descended into its depths, farther and farther away from the rays of the sun.

Trot was surprised to find she could see so plainly through the high wall of water above her, but the sun was able to shoot its beams straight down through the transparent sea, and they seemed to penetrate to every nook and crevice of the rocky bottom.
In this deeper part of the ocean some of the fishes had a phosphorescent light of their own, and these could be seen far ahead as if they were lanterns.

The explorers met a school of argonauts going up to the surface for a sail, and the child watched these strange creatures with much curiosity.

The argonauts live in shells in which they are able to hide in case of danger from prowling wolf fishes, but otherwise they crawl out and carry their shells like humps upon their backs.

Then they spread their skinny sails above them and sail away under water till they come to the surface, where they float and let the currents of air carry them along the same as the currents of water had done before.