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.
3. The Stellerides, or Asterias, are the star-fish. The mouth in these creatures is in the middle of the lower surface, and it has a membranous lip, capable of great dilation, but furnished with angular projections for capturing its prey. The skin is soft, but leathery, and it is covered on the back with spongeous tubercles, or scales. The rays are hollow beneath, and furnished with tentacula, by the aid of which the star-fish manages to crawl backwards, forwards, or sideways, as the case may be, any of the rays serving as a leader. These animals are found on the sea-shore, forming large beds, which are washed over by the sea. The Crinoidea, or stone-lilies, of which such curious fossil specimens have been found, are nearly allied to the star-fish.
II. The Intestina, or Entozoa. The intestinal worms were divided into two kinds by Cuvier, viz. the Cavitaires, including the worms of children, and other cylindrical worms; and the Parenchymateux, or flat worms; such as the fluke in sheep and the tape-worm in human beings. The Entozoa are now universally regarded as belonging to the Articulated or Annulose division of the animal kingdom.
III. Acalephæ, or Sea-Jellies. These creatures are of a soft and jelly-like substance, with a thin skin, and an unarmed mouth. The Medusides are very numerous, and produce that beautiful phosphorescent light noticed by voyagers in the Australian seas. The most interesting of the Acalephes is the Portuguese man-of-war, or Physalia.
IV. Polyps, or Anthozoa, according to Cuvier, were divided into three orders; namely:
1. Fleshy Polyps (Sea anemones);
2. Gelatinous Polyps (Hydra); and