The sexes are usually separate. Sexual reproduction typically consists of releasing eggs and sperm into the water, with fertilization taking place externally. Their larvae are planktonic.
Many echinoderms have remarkable powers of regeneration. A starfish arm cut off will regenerate. A section as small as a single arm, with some central disk and neural tissue, can regenerate the whole organism.
Most of the adults have radial symmetry. If they are not sessile or attached, they are at least benthic (bottom-dwellers). In contrast, echinoderm larvae are planktonic, and have bilateral symmetry. Echinoderm larvae are ciliated free-swimming organisms that have a bilaterally symmetry rather like embryonic chordates. The first stage in all groups except the crinoids is the dipleurula, which has a row of cilia winding round its body. All the groups have second and third stage larvae. The larvae grow, and are carried away from their parents.
Later, the adult body grows into a radial form, with the body arranged in five parts around a central axis. This means they often have a star-shaped pattern.
There is no geological evidence about the origin of echinoderms. The first undoubted fossils in the Lower Cambrian are already well developed, and fall into five or six different groups. "Arkarua", from the Ediacaran, is thought to be an echinoderm. "Tribrachidium" is another possible, from the same period. On general biological grounds, it seems that echinoderms, and some related phyla, may have been derived from worm-like ancestors.
Of all invertebrate phyla, the echinoderms are the closest relatives to the chordata, that is, animals who have spinal cord, such as vertebrates. With the chordates and hemichordates, echinoderms are deuterostomes, one of the two major divisions of the animal kingdom.
Argonaut
The word Argonaut might refer to:
Das Rheingold
Das Rheingold ("The Rhinegold") is an opera by Richard Wagner. It is the first of the four operas that together tell a story called "Der Ring des Nibelungen" ("The Ring of the Nibelung").
"Das Rheingold" is an opera in one act, divided into four scenes. It is a very long act, lasting 2 ½ hours, but it is normally played without an interval as the music is continuous, even between the scenes.
The characters in the opera are Gods, Giants, Nibelungs and the Rhinedaughters. The Nibelungs are a race of dwarfs. They are cunning and evil and live underneath the surface of the earth.
The first scene takes place in the river Rhine where the three Rhinemaidens: Woglinde, Wellgunde and Flosshilde, are guarding the precious Rhine gold. Alberich, an evil Nibelung, arrives and tries to catch the Rhinemaidens. From what they say Alberich learns that the gold they are guarding has magic powers. Alberich wants the gold, but the Rhinemaidens say that anyone who wants to have it has to renounce (give up) love. They think that Alberich will not do that because he is very fond of girls, but Alberich does renounce love and so he gets the gold.
The second scene takes place in Valhalla, which is where the gods live. Wotan, the chief god, is there with his wife Fricka. Two giants have been building an enormous castle for the gods. Wotan has promised them that he would pay them for their work by giving them Freia, the Goddess of Youth and Beauty. Fricka is worried about the promise that Wotan has made, but Wotan tells his wife that he is not going to keep his promise. He thinks he can find a way of tricking the giants. Freia enters. She is frightened because the giants are coming for her.
The giants, Fasolt and Fafner, enter. They want to have Freia in payment for having built the castle. The problem for the gods is that Freia has a garden in which she grows golden apples. If the gods no longer have these golden apples they will grow old and die. Wotan does not know what to do.
Loge, the god of fire, enters. Wotan is cross with Loge for not having found something else to pay the giants with. Loge says he has been all over the world looking for something. He tells everyone that the gold from the Rhine has magic power, and how Alberich renounced love so that he could steal it from the Rhinemaidens. He tells them that a ring can be made from the gold, and that whoever has the ring will have power over all the world. The giants hear what Loge is saying and decide that they want the gold instead of Freia. They take Freia as hostage, saying that they will be back later to collect the gold. If they do not get it then they will keep Freia forever.
Wotan asks Loge how they are going to get the gold. Loge tells them that they should steal it from Alberich. After all, Alberich stole it from the Rhinemaidens, so it does not belong to him. Wotan and Loge go down below the earth to the Nibelungs.
Underneath the earth Alberich is forcing the Nibelungs to work for him. They are hammering at the anvil. Mime, who is Alberich’s brother, has used some of the gold to make a helmet. This helmet is called the Tarn helmet. Anyone who wears it can change their shape or become invisible. Alberich treats his brother very cruelly.
Wotan and Loge tell Alberich that they have heard how clever he is. They ask him to show them what he can do with the Tarn helmet. Alberich puts on the helmet and changes himself into a huge dragon. Wotan and Loge pretend to be impressed. They ask him whether he can change himself into something small. Alberich show off again, changing himself into a frog. Wotan immediately puts his foot on him and catches him. Wotan and Loge take all the gold and go back to Valhalla.
Wotan and Loge tease Alberich. Alberich asks them what they want so that he can be free. Wotan demands the gold. The Nibelungs pile the gold up. Alberich is not too worried, because he still has the ring which gives him magic power. He thinks he can use this power later to get the gold back. Wotan sees the ring on Alberich’s finger. He tells him to give him the ring. Alberich is terrified, but Wotan tears it from his finger. Before Alberich goes he puts a curse on the ring. Whoever has had the ring will die. This is very important to the whole story of the four operas.
The two giants appear with Freia. The gods have become very weak, but once Freia returns they start to become stronger again. Wotan tells the giants they can have their gold. The giants say that they must pile up the gold in front of Freia until they cannot see her. They do this, but the giants say there is still a tiny hole through which they can see Freia’s eye. They see the ring on Wotan’s finger and tell him to put it in the hole. Wotan says they can have anything they want but not the ring.
Erda appears. Erda is the Earth Goddess. She is very wise. She reminds Wotan that there is a curse on the ring and that he should quickly give the ring to the giants. Wotan has no choice. He puts the ring in the hole. Now the giants are happy.
Immediately, however, the giants start to argue with one another and Fafner kills Fasolt. The curse on the ring is working. Loge tells Wotan he was wise to give up the ring. However, Fricka is still worried because Wotan did have the ring.
Donner, the god of Thunder, enters and swings his hammer. There is a flash of lightning and a crash of thunder. The clouds vanish, a rainbow makes a bridge to Valhalla and the gods enter Valhalla. The Rhinemaidens are heard crying out to Wotan to give the gold back, but Wotan takes no notice. The gods are now at the height of their glory. We shall see, at the end of the fourth opera, that the curse will catch up with them and that they, too, will die.