EchinodermataEchinodermata, one of the Phyla, or main groups of the animal kingdom. It includes the Sea-cucumbers, Sea-urchins, Starfish, Brittle-stars, Sea-lilies, and some less-known extinct classes. The name is derived from the best-known member of the group, the common Sea-urchin (Echinus esculentus), and means "spiny-skinned." This term is not, however, appropriate to some groups, such as the Sea-cucumbers, in which there is either no skeleton or only a few thin, scattered plates, or spicules. The one character which all the Echinodermata possess in common is a series of canals, tubes, or reservoirs that convey water throughout the body. This is known as the "water-vascular system." Its function is either respiratory (Sea-lilies), locomotive (most "regular" Sea-urchins, including the common British species), or both (as in the Heart-urchin). In addition to the water-vascular system, there is a series of vessels through which flows the blood. This is, therefore, called the blood-vascular system. It serves to distribute the food after its digestion in the stomach. As the blood is not definitely corpusculated, the so-calied "corpuscles" having very irregular forms, the blood has been called the "chylaqueous fluid." The nervous system is very primitive in character; it consists of a series of nerve cells scattered about the muscular tissue; processes from these cells may unite with one another to form a loose nerve-plexus, or cord. The nerves are usually in the superficial layers of the body. The digestive system differs from that of the Coelenterata (q.v.) by being closed and completely separated from the body cavity. There is a mouth generally opening in the centre of the oral or abactinal surface. This leads by a short oesophagus to a simple but fairly capacious stomach, or "gut." The gut in the Sea-lilies and extinct Mastoids and cystoids was a wide coiled tube. The intestine is short, and may open to the exterior by an anus; in many members of the group, however, there is no anus. From the stomach five pairs of pouches arise, and serve to store and digest food; these are known as hepatic caeca. The nervous, blood, and water-vascular systems of the Echinodermata are all arranged fundamentally on the same plan. Each has a ring round the oesophagus, from which five branches radiate. The animal is, therefore, composed of five similar portions, or "actinomeres." These may be more or less completely marked of arms, as in the Starfish, Brittle-stars, and Sea-lilies, or of five segments, as in the Sea-urchins and Sea cucumbers. The development of the Echinodermata is marked by a remarkable metamorphosis, the details of which differ greatly in the different classes. Reference should be made to each of these. The Echinodermata are all marine, except a few forms which live in brackish water. They have a great range in space, depth, and time. Owing to the strength and importance of their skeletal structures, they are often preserved in a fossil condition, and are of great value to the geologist. The Echinodermata may be grouped into classes: -
I. Cystoidia. All extinct.
II. Blaistoidea. All extinct.
III. Crinoidea, or Sea-lilies.
IV. Ophiuroidea, or Brittle-stars.
V. Asteroidea, or Starfish.
VI. Echinoidea, or Sca-urchins.
VII. Holothuroidea, or Sea-cucumbers.