10. Thysanoura, or spring-tail insects. These creatures are of small size, and without wings; they are found in crevices of woodwork, or under stones. The principal genera are Lepisma and Podura.

11. Parasita, or parasitical insects, such as the louse. They are also without wings.

12. Myriapoda. This order is made a separate class by many naturalists, as the creatures contained in it are distinguished from the true insects by the great number of their feet; by the want of distinct divisions into thorax and abdomen; and by the great number of segments into which the body is divided. The principal insects in this order are included in the Linnæan genera Julus and Scolopendra, commonly called centipedes.

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

3. Polyps with Polyparies, the latter including all the various compound zoophytes, with the Sponges. Of these the Flustræ, or Sea Mats, and numerous allied species, have since been recognised as belonging rather to the Mollusca, and the Sponges to a distinct and lower group of animals than the Radiata; the remainder have generally been divided into the following three orders:—

1. Helianthoida. This order includes the actinia, or sea-anemone; and the madrepores, sea-mushrooms, and brainstones, which live in communities, and possess the power of secreting calcareous matters, which they emit to form these stony substances.

2. Asteroida. Some of the animals belonging to this division are called sea-pens, and others form some of the different kinds of coral, particularly that used for necklaces, &c.

3. Hydroida. This order includes the fresh-water polypi, which, it is well known, by the experiments that have been tried, may be cut in pieces and even turned inside out without destroying life. It must be observed that the contents of this group in Cuvier’s system consisted of all those forms of animals which he could not, in accordance with the knowledge possessed in his day, conveniently place anywhere else. Within the last few years, however, great progress has been made in the arrangement of the animals placed in this group by Cuvier. One of the most important changes has been the establishment of a fifth group of animals for the Infusoria and Sponges, together with certain other creatures of very low organisation. To these the name of Protozoa has been given. The Entozoa have been removed amongst the articulate animals, and there is a growing conviction that the Echinodermata will have to be transferred to the same section. There remain, consequently, the Acalephæ and Polyps of Cuvier, which form a group characterised by their soft and generally gelatinous texture; by the existence of peculiar cells, called thread cells, in the skin; and by their possession of an alimentary cavity with only a single orifice. To these the name of Cœlenterata has been given. They are divided into two classes: I. The Anthozoa, or Polyps, including the orders Helianthoida and Asteroida; and II. The Hydrozoa, composed of the Hydroid Polyps and Acalephæ, the connection between which, as indicated in the text (p. 609), is very intimate.

V. The Infusoria, or Animalcula, are so small as to be invisible to the naked eye, and they are all inhabitants of liquids. Cuvier arranged them in two orders, one of which he called Les Rotifères, and the other Les Infusories homogènes, but the first of these divisions is now included among the Articulata. The remainder of the Infusoria of Cuvier, with the exception of some which are now known to be of vegetable nature, are arranged, with the Sponges and some other animals, in a separate division, called Protozoa, the classification of which is still in a somewhat uncertain state. The three principal classes are those of the Infusoria, the Sponges, and the Rhizopoda; but there are other forms which will not admit of being brought under any of these denominations. Nearly all the Protozoa are microscopic, except when, as in the case of the Sponges, they form an aggregation of individuals. They are very numerous, and, although exceedingly simple in their structure, their history often possesses much interest.