Are so called because their organs of locomotion, and even their internal viscera, are generally arranged in a circle round a centre, so as to give a radiated appearance to the whole body. The animals included in this class are the very lowest in the scale; they have scarcely any external senses; their movements are slow, and almost their only sign of life is a craving for food. Some of them, however, have a distinct mouth and alimentary canal, with an anal orifice; others have a bag-like stomach with a kind of mouth, through which they both take their food and reject their excrements; while others have no mouth, and appear only to absorb nourishment through pores. In the like manner, though some are oviparous, others may be propagated by division into plants. Of these Cuvier makes five classes: I. _Echinodermata_, or sea-urchins. These animals have a leathery or crustaceous skin or shell, commonly covered with numerous tubercles. The mouth is generally in the centre of the animal, and is often armed with five or more pieces of bone, which serve as teeth; the stomach is a loose bag; the organs for respiration are vascular; and the animals are oviparous. They are furnished with tentacular tubes, which serve as arms or feet, and which they can push out and draw back at pleasure; and they have yellowish or orange-coloured blood, which appears to circulate. Cuvier divides this class into those with feet, and those without; but Lamarck, whose arrangement has been more generally followed, divides them into three orders; viz.: 1. _The Fistuloides_, or _Holothurida_, which have cylindrical bodies, leathery skins, and mouths surrounded by tentacula. These creatures live in the sea, or in the sands on the sea-shore; the trepang, or eatable worm of the Chinese, is one of them. 2. _The Echinides._ These are the sea-urchins, properly so called, and the shells, when the animals are out of them, are called sea-eggs. The Echinides live in the sea. They lay eggs, and the roe, or imperfect eggs, occupy a large portion of the space within the shell when the animal is still alive.