As the woods were full of tree spirits, so the sea was full of water spirits, who were called Nereids. One of the most beautiful of these was named Amphitrite. She lived with her father and mother in a grotto under the waves. Their home was charming with coral and seashells, and a pretty garden of seaweeds was before the door.

Poseidon had heard of this lovely creature, so he called a dolphin, upon whose back he mounted and rode off to make a visit. He found the nymph very pleasant and agreeable, and she asked him to call again. After a few visits he asked her father if she might be his wife.

The father was entirely willing. When they asked Amphitrite, she said "The mighty ruler of the seas does me too much honor, but I will gladly take him for my husband."

Poseidon was greatly pleased. He said to his dolphin, "Good fish, you must carry double to-day. My bride goes home with me this afternoon, and you shall have a shining reward."

They arrived in safety, and Poseidon said, "Now, faithful dolphin, live no more under the waters, but shine in the sky among the stars."

The happy couple under the sea had a son, called Triton. He went before his father, blowing a trumpet, whenever the king of the sea rode out.

Poseidon did not ride on dolphins any more. He created horses, and harnessed four of them to his chariot. These had hoofs of brass and manes of gold. When the king was going out to drive, Triton blew his horn. The waters became as smooth as glass. Triton went before, the prancing horses drew the chariot, and whales and other huge dwellers in the deep followed after.

Poseidon had another son, called Proteus, who was very wise, and could foretell the future. He could change himself into any shape, and be a seal, a whale, a fish or a man, at his pleasure.

Thetis was another Nereid, and sister of Amphitrite. She married a man, Peleus, and their son was the great Achilles, of whom we shall learn more when we read of the Trojan war.

An insane husband once chased his wife who was carrying their little boy. Coming to the edge of a cliff which overhung the sea, in her fright she leaped into the water. The gods who were sorry for her, changed her into a sea-goddess, called Leucothea, and her son into a god named Palæmon. He generally rode upon a dolphin. Sailors in danger of shipwreck prayed to him and his mother, and they often answered such prayers, for they remembered how kind the gods had been to them.

Every river and every fountain had guardian spirits. These were called Naiads, and are shown in pictures as beautiful young women.

They were generally friendly to mankind, but sometimes they carried their favorites away from home and family. So they did with Hylas. He was a very good-looking lad, who, being in a strange country, went to a spring for water.

One of the Naiads saw him, and called to the others, "Oh, come, see this pretty boy!"

He bent down to fill his pitcher, but saw sweet faces jumping up out of the water.

"Oh, pretty boy!" they said. "Do you live near here? We never before saw you at this spring."

Hylas blushed and said, "No, I am a stranger, and must hurry back to the ship and my friends."

"Oh, no!" they said. "Do not go away. Stay with us!"

They stretched up their arms out of the water, caught Hylas, and pulled him down into the spring. His friends afterwards found the empty pitcher by the waterside, but Hylas was never seen again.

Arethusa was a wood nymph who was fond of hunting. One day, warm from the chase, she stepped into a river to cool herself, but a voice spoke to her. She was frightened, and tried to run away. The voice said, "Why do you fly? I am Alpheus, the god of this stream!" That made her run the faster. The god followed her, and she prayed to Artemis, who changed her into a river which sank into the ground and came out again far away in Sicily.